I went searching for an answer before asking this, but I wasn't able to find an answer so I am hoping you all can point me in the right direction.
I am using 1.8 in a single survival mode game. I have a item sorting system setup and have a row of sorting droppers within that system. The problem I have is that some of the sorters (not all) do not retain the items I put in them. (18-1-1-1-1) These are normal stackable items like coal, iron, cobblestone, etc...
The redstone is identical on all of the sorting modules, so I am at a bit of a loss as to what is going wrong.
Did you mean sorting "Hoppers" rather than droppers?
If they aren't holding the sort items, then there must be some sort of redstone signal leakage.
Does the problem happen all the time? The only thing I can think of is that if you dump a large amount in (say 46 of the item you want to sort, bringing the 18 up to 64) it could send a signal out that will activate neighbouring cells, causing them to drop their items and breaking the sorter. There are designs to overcome this though.
I've used the following design and am really happy with how it works for me:
(Minecraft - Item Sorter Overflow Fix - with Marsman)
The most likely issue is that the hoppers are becoming overloaded by "stacks" of items, which causes the redstone to get over-powered and affect other nearby hoppers. This screws everything up. If you're using water streams to deposit the items into your sorter hoppers, you want to use a "buffer" hopper to feed those items into the chain individually rather than allowing them to drop directly into the sorter hoppers as grouped stacks of items.
The problem with trying to use a buffer hopper for high volume water stream item sorters is that the buffer hopper would also need to be set up to only accept the sorted items. For the water stream sorter to work, the hopper under (or beside in the case of an ice based water channel) has to be the sort hopper, otherwise the items will be sucked up into the first empty hopper. Of course, you could set it up as a secondary hopper chain that runs in parallel with the water stream, but that would effectively negate the speed boost of using the ice channels, and would probably introduce considerable lag with the large chain of hoppers.
You can set up the sort hoppers so that they allow more items to be sucked in (using a 2-5-5-5-5 setup, assuming that your filler item is abundant enough to do that for all of your hoppers (I'm using dead bushes in mine, but only currently have enough for 18-1-1-1-1 setup, as it's a 128 bay sorter and it took long enough to gather 8 stacks of Dead Bushes). That would allow more items to go through without filling up. Though, without using the overflow protection that I linked above, you would still have the issues of signal creep, which could destroy the sorter set up.
For my system, I use droppers to feed items into the system. They are set so that they will only spit out items as fast as a hopper can process them, so, in theory, none should be missed due to overflowing the sorter hoppers. It does happen on occasion, so I have a few overflow chests and just send the missed items for re-processing every so often.
I don't know if this applies, but I put in a sorter in a zombie pigmen farm in the nether and it was missing items - it sorted most gold and rotten flesh just fine most of the time, but some would slip past the filters and end up in the dump chest at the end of the hopper "stream". I tried all sorts of things and couldn't eliminate the leakage. Then it suddenly dawned on me. The Zombies were dropping onto a 3x3 platform of hoppers and died and dropped their loot. I had them all fed into a single hopper which took the loot down one block level, then I had a chain (stream) of hoppers run right under the killing platform. The filter hoppers were on the next layer down from that (as you would expect). The problem was that when an item in a filter was in the stream hopper above it and also had just be collected by a killing hopper right above that, I ended up with two filters on top of each other and the items would drop down into the stream hopper. That hopper couldn't handle two (or more) items at once, so it passed them on down the stream and they didn't get sorted.
So, the moral of the story is, make sure you have a non-hopper in the block above the stream of hoppers in your sorter. In fact, if you don't want hopper "cross talk" you should avoid running a horizontal chain of hoppers right over another hopper(s). It's obvious to me now, but I was beating my head against the wall for two days trying to figure out what was going on!