Mobs don't spawn - Server platform

Put the summary of the bug you're having here

What I expected to happen was...:
Mobs spawn more frequently, in mob farms and holes artificialy made 0 mob spawn, and at night time and nether fortres like 2-3 mobs every 5 minutes. Y coord was no multiple of 16.

What actually happened was...:
Mobs spawn is reduced dramaticaly and some times to 0