You are playing on a server (since it says "Spigot server" at the top) and ALL players must spawnproof the entire area round them - if even one player likes to e.g. go caving instead of farming (like myself) then they will have mobs spawning around them, filling up the global mob cap (the game does adjust the mob cap based on the number of chunks loaded by all players but there is only one global cap for all players). The fact you are on a Spigot server is another issue; as an optimization it reduces the mob cap, entity spawn range, and mob spawn rates in a way that especially hurts mob farms (in my own experience, by optimizing the game by reducing per-chunk spawn rates, reducing spawn rate has little if any effect on natural mob spawning on the surface at night or in caves since mobs can spawn far faster than they despawn but it will hurt mob farms since they depend on the spawn rate).
I cant figure out why no hostile mobs are spawning in my grinder, it is in a constant state of having around 0/10-20 entities on my f3 entity counter
You are playing on a server (since it says "Spigot server" at the top) and ALL players must spawnproof the entire area round them - if even one player likes to e.g. go caving instead of farming (like myself) then they will have mobs spawning around them, filling up the global mob cap (the game does adjust the mob cap based on the number of chunks loaded by all players but there is only one global cap for all players). The fact you are on a Spigot server is another issue; as an optimization it reduces the mob cap, entity spawn range, and mob spawn rates in a way that especially hurts mob farms (in my own experience, by optimizing the game by reducing per-chunk spawn rates, reducing spawn rate has little if any effect on natural mob spawning on the surface at night or in caves since mobs can spawn far faster than they despawn but it will hurt mob farms since they depend on the spawn rate).
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?