Custom mobs not spawning

I have 3 custom mobs so far and one of them is set to ambient with default spawn rate and spawns fine. The other two are creature spawns (One has an aggressive AI and the other is passive) but neither seems to spawn even with >20 spawn rate. The mobs work fine when spawned in by spawn egg, and the aggressive creature used to spawn fine before but now it doesn't and I'm not sure why.