Mobs not spawning?

[thefourtunewookie]

Doing a modded playthrough and things were completely normal at first, but recently very specific mobs do not spawn at all naturally (atlatitan, hullbreaker, vesper, and tremorsaurus in my experience. They can be spawned with spawn eggs mind you) and I cannot figure why. I deleted and reimplemented the config folder but it is still bugged (not that I changed anything much with the config to begin with). Unsure what to do.