I wanted to have this as only spawning area for wither skeletons. Button placed all over the place. Now, nothing is spawning anywere. Even when I left and came back later, only 1 blaze spawned on that netherrack without wither roses...
You should make an afk area at spawnable distance from the farm and then see whether it works or not. Also you have place a lot of buttons all over the fortress to make the farm more effective but still check around if there are other places where other mobs are spawning and the skeletons are not able to spawn because of the other mobs.
Nether mobs have a 2-part spawning mechanism. Firstly, just like overworld hostile mob spawning, you have the "normal" hostile-mob cap of 70 mobs and the associated spawn conditions (must be solid block, must have enough room to fit the mob, must not be a wither rose if not a wither skeleton, must be light level 11 or lower for blazes, etc). This is what the buttons and other spawnproofing methods are manipulating, as if they cannot spawn "over there" then the only place they can spawn is "in here (ie, your platform)". This is how you traditionally increase the spawning rate of your grinder.
In 1.16.2, I think, a mob-type spawnproofing sphere was enacted around certain nether mobs. These mobs are: ghasts, endermen (note, this only applies to endermen in the nether, not to endermen in the other dimensions), blazes, wither skeletons, regular skeletons, and magma cubes. It probably also applies to the zombie/regular piglins and hoglins...but I'm unsure about these. What this means that when one of these mobs spawns, a second mob of that type cannot spawn within a certain distance of it (I think it's somewhere between 8 and 16 blocks). Furthermore, as such mobs wander around, when multiple mobs get close enough that their spheres overlap the anti-spawning sphere is redrawn to encompass all individuals via a common centerpoint--this ensures that any random gaps in-between each individual mob where the spheres might not overlap are filled in and therefore the enforced density of mob spawns is retained. This mechanism was implemented to fix unintended spawning mistakes in the new nether biomes that the older spawning system was unable to handle
Wither skeletons are very particular about where they spawn unless things have changed in recent updates.
It's possible that the towers are too high up, you could try building the spawning area at the same level as the walkways.
Also I believe there are 2 types of spawning areas in a nether fortress, on the walkways and in the corridors they will spawn on any solid blocks, between the walkways they only spawn on nether brick, I've only ever seen this in reference to walkways and I don't know if the towers count as the first type or not.
So try building it lower and using nether brick for the floor.
Wither skeletons are very particular about where they spawn unless things have changed in recent updates.
It's possible that the towers are too high up, you could try building the spawning area at the same level as the walkways.
Also I believe there are 2 types of spawning areas in a nether fortress, on the walkways and in the corridors they will spawn on any solid blocks, between the walkways they only spawn on nether brick, I've only ever seen this in reference to walkways and I don't know if the towers count as the first type or not.
So try building it lower and using nether brick for the floor.
It is same height as it was before - 2 block high gap on both sides. But I will try to put nether brick there and see what it does.
Edit: tested it, now it spawn, but what about those other mobs that I don't want there?
Another edit: It spawn really rarely, went completly away and back, its like 20% of spawn anything in there.
but what about those other mobs that I don't want there?
Use minecarts or boats to capture the mobs you don't want to spawn there. Nametag them so they don't despawn nor count against the mob cap, and position them so that their anti-spawning areas cover the spawn platform. If you use minecarts, you can make small rail lines that move the mobs far enough away to allow those mobs to resume spawning.
Use minecarts or boats to capture the mobs you don't want to spawn there. Nametag them so they don't despawn nor count against the mob cap, and position them so that their anti-spawning areas cover the spawn platform. If you use minecarts, you can make small rail lines that move the mobs far enough away to allow those mobs to resume spawning.
all I wanted is wither skeleton farm for skulls. I don't want any other mobs to even live.
Wither skeletons are very particular about where they spawn unless things have changed in recent updates.
It's possible that the towers are too high up, you could try building the spawning area at the same level as the walkways.
Also I believe there are 2 types of spawning areas in a nether fortress, on the walkways and in the corridors they will spawn on any solid blocks, between the walkways they only spawn on nether brick, I've only ever seen this in reference to walkways and I don't know if the towers count as the first type or not.
So try building it lower and using nether brick for the floor.
According to the Wiki this is still the case; fortress-specific mobs can spawn on any valid spawnable block within the bounding box of an individual piece (any type) but only on nether brick within the bounding box of the entire fortress:
In Java Edition, the spawning algorithm has two checks:
It checks if the spawn coordinates are within the "bounding box" of a single piece (e.g. corridor or walkway) of the fortress. In this case the block type of the ground does not matter.
It checks if the spawn coordinates are within the "bounding box" of the entire fortress and whether the ground consists of nether bricks.
Basically, as long as the ground is nether brick they should be able to spawn as long as you are within the bounds of the entire fortress (a cuboid whose dimensions encompass the min-max xyz coordinates, so an "L" shaped fortress would be able to spawn mobs in the corner without any actual structure, and between the altitudes of the lowest floor and highest roof (the support pillars do not count).
I wanted to have this as only spawning area for wither skeletons. Button placed all over the place. Now, nothing is spawning anywere. Even when I left and came back later, only 1 blaze spawned on that netherrack without wither roses...
A similar bug was posted on the official bug site which was resolved.
https://bugs.mojang.com/browse/MC-203335
You should make an afk area at spawnable distance from the farm and then see whether it works or not. Also you have place a lot of buttons all over the fortress to make the farm more effective but still check around if there are other places where other mobs are spawning and the skeletons are not able to spawn because of the other mobs.
Nether mobs have a 2-part spawning mechanism. Firstly, just like overworld hostile mob spawning, you have the "normal" hostile-mob cap of 70 mobs and the associated spawn conditions (must be solid block, must have enough room to fit the mob, must not be a wither rose if not a wither skeleton, must be light level 11 or lower for blazes, etc). This is what the buttons and other spawnproofing methods are manipulating, as if they cannot spawn "over there" then the only place they can spawn is "in here (ie, your platform)". This is how you traditionally increase the spawning rate of your grinder.
In 1.16.2, I think, a mob-type spawnproofing sphere was enacted around certain nether mobs. These mobs are: ghasts, endermen (note, this only applies to endermen in the nether, not to endermen in the other dimensions), blazes, wither skeletons, regular skeletons, and magma cubes. It probably also applies to the zombie/regular piglins and hoglins...but I'm unsure about these. What this means that when one of these mobs spawns, a second mob of that type cannot spawn within a certain distance of it (I think it's somewhere between 8 and 16 blocks). Furthermore, as such mobs wander around, when multiple mobs get close enough that their spheres overlap the anti-spawning sphere is redrawn to encompass all individuals via a common centerpoint--this ensures that any random gaps in-between each individual mob where the spheres might not overlap are filled in and therefore the enforced density of mob spawns is retained. This mechanism was implemented to fix unintended spawning mistakes in the new nether biomes that the older spawning system was unable to handle
Wither skeletons are very particular about where they spawn unless things have changed in recent updates.
It's possible that the towers are too high up, you could try building the spawning area at the same level as the walkways.
Also I believe there are 2 types of spawning areas in a nether fortress, on the walkways and in the corridors they will spawn on any solid blocks, between the walkways they only spawn on nether brick, I've only ever seen this in reference to walkways and I don't know if the towers count as the first type or not.
So try building it lower and using nether brick for the floor.
Just testing.
It is same height as it was before - 2 block high gap on both sides. But I will try to put nether brick there and see what it does.
Edit: tested it, now it spawn, but what about those other mobs that I don't want there?
Another edit: It spawn really rarely, went completly away and back, its like 20% of spawn anything in there.
Use minecarts or boats to capture the mobs you don't want to spawn there. Nametag them so they don't despawn nor count against the mob cap, and position them so that their anti-spawning areas cover the spawn platform. If you use minecarts, you can make small rail lines that move the mobs far enough away to allow those mobs to resume spawning.
all I wanted is wither skeleton farm for skulls. I don't want any other mobs to even live.
According to the Wiki this is still the case; fortress-specific mobs can spawn on any valid spawnable block within the bounding box of an individual piece (any type) but only on nether brick within the bounding box of the entire fortress:
Basically, as long as the ground is nether brick they should be able to spawn as long as you are within the bounds of the entire fortress (a cuboid whose dimensions encompass the min-max xyz coordinates, so an "L" shaped fortress would be able to spawn mobs in the corner without any actual structure, and between the altitudes of the lowest floor and highest roof (the support pillars do not count).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?
You would rather take the time to place down 43 chests' worth of pressure plates, buttons, etc than to capture and nametag maybe 10 mobs?