Why can't a spawn specific mobs in a creative server?

[CaptainArgus]

I recently created a creative server where my friends and I can work on Redstone, and have noticed that I can't spawn in certain mobs into the world. I've only seen 2 mobs I couldn't spawn which were shulkers and pufferfish, but there might be more. I am also capable of spawning in all sorts of other mobs, including skeletons, creepers, zombies, boats, end crystals, hopper minecarts, and elder guardians (these are the ones I tested).

I've tried spawning them in using both commands and spawn eggs, but neither works, and nothing shows up in chat/console. I've also tried spawning in different dimensions, changing the difficulty, and turning on mob spawning, but none of those made it possible to spawn these entities.

The only plugins I have installed are worldedit are the worldedit particles plugin.

Any ideas why this might be happening?

[CaptainArgus]

Ive now tried converting the server from paper to spigot and nothing changed. I really have no clue what could be wrong.

[overlordnikoman]

we need help