Also, do you have Ars Magica installed? As in, is your world also filled with Hecates who do nothing but stand around and do nothing? If so, well, there is your problem. Sometimes, for no reason all, Hecates will break. They will spawn at the exclusion of EVERYTHING else. No hostile mobs, no animals, no neutral mobs, only Hecates. In every dimension, too.
Man, THAT was an annoying problem to track down the first time it happened. "Why is nothing spawning?" "Wait, I still have Hecates standing around, I wonder...." /cofh killall *1 thousand Hecates die* everything starts spawning for ten minutes, 1 thousand Hecates spawn, no new spawns "Damnit", chucks Ars Magica, everything goes back to normal.