Check the nearby Chunks for mobs. You are probably at the 30 mob limit. If you get the normal Spawner animations but no mobs, this is also a a likely indication you are at the Mob Spawn Limit for that 3x3 chunk area
Best place i found for a mobfarm was in the last milennium. Just claim the areas you put them in and spread them out with at least 1 empty chunk in between farms. You can have several in a rather small area without ever hitting the mobcap. It also rarely bugs out in there.
you might just have to unload the chunk. if that doesnt help, wait untill a restart and then check back.
It started working when I logged back in, and it is still working now. I'll try taking away the chunkloader and relogging if it stops in the future.