Mobs not spawning

Hello! I need a little bit of help.

For my mod, I have added in LOADS of new mobs (60+ as of right now). They all used to spawn correctly and in their proper biomes (They have the behavioral characteristics of mobs, but the spawning characteristics of creatures). But now, none of them spawn, aside from one or two in the chunk I spawn in at, then no more spawn at all. Does anyone know what causes this?