Affects Version/s: Minecraft 1.13.1
Fix Version/s: None
Environment:java version "1.8.0_111"
Java(TM) SE Runtime Environment (build 1.8.0_111-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.111-b14, mixed mode)
Basically, when there are a ton of zombie pigmen, blazes, and/or ghasts in an area of a nether fortress, nether fortress mobs, like skeletons and wither skeletons will not spawn unless you kill a lot of the other mobs. This makes it very difficult to farm for wither skeleton skulls without either manually killing the other mobs or using /kill @e[type=____].
What I suspect is happening is that there is a max monster limit in an area and all of the pigmen and ghasts are taking the positions of the wither skeletons and regular skeletons that spawn in nether fortresses.
How to replicate:
Go to a nether fortress.
Kill all of the spawned wither skeletons
To see that they are not spawning do /say @e[type=wither_skeleton]
Run around the fortress and keep typing that command.
Once the entire fortress has been generated and all of the skeletons have been killed, they will no longer spawn unless the other mobs are despawned and/or killed.
Give wither skeletons, skeletons and magma slimes their own "place holders" in the mob spawning system so that they will spawn even when the pigmen and ghasts are spawned. By having seperate spawn caps for both sets of mobs, both sets of mobs can spawn freely without one dominating over the other (pigmen not letting other mobs spawn).