When i tried to spawn a hostile mob with a spawn-egg, it disapeared. Also hostile mobs won't spawn naturaly

Put the summary of the bug you're having here

What I expected to happen was....
Describe what you thought should happen here

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...