Entities Refuse to Spawn

Several of my custom entities refuse to spawn, and I don't understand why.

Basically, what happens is, when I try to spawn the entity via Spawn Egg or /summon, it just instantly disappears.

I've tried everything I can think of, blockbench issues, despawning when idle, etc.

However for some reason, whenever I Run the server and try spawning it there, it works perfectly fine.
But as soon as I join a singleplayer world, it'll just disappear.

Another odd thing, when I add an entity base to the entity, it'll spawn, but of course, it wont behave as intended because of the entity base.

I'm obviously failing to find out how to do this myself, so any help would be greatly appreciated.