Mob Spawn Rate Extremely Low

Mob Spawn Rate Extremely Low

What I expected to happen was...:
Mobs to spawn at night and in dark places.

What actually happened was...:
At Night and in dark places, there are barley any aggressive mobs, I've tried
Easy, Normal, and Hard. Still barley an spawns. I went adventuring in the night and saw about 8 mobs the whole night and it was completely dark.