No Monsters spawn on normal

I don't have any hostile mobs in my survival world!

I play on normal.
If I load my world, there are no hostile mobs anywhere!!
If I pause the game and put it on default and back on normal, then sometimes some mobs spawn.
No more mobs spawn after that.

• Spawners work, they spawn mobs.
• In the nether occasionally some mobs spawn (of all kind).
• Sometimes a slime spawns in the overworld

I don't know what to do to reproduce the bug... Sorry.

Help please! I want to kill mobs!
And have a great new year! :-D