Put the summary of the bug you're having here

What I expected to happen was...:
Nether mobs to spawn.

What actually happened was...:
No Nether mobs spawn except for Blaze.

Steps to Reproduce:
1. I turn on easy difficulty
2. Go into the nether
3. And nothing is spawning. No pigmen, Ghasts, wither skeletons, magma cubes. Only Blaze at their cage