So I'm sure this has been posted before, and a lot of them claim to be solved, but have no real answer on how they get solved. My friend joined my world and it seems like no enemy mobs spawn near him. Mobs for me spawn regularly and spawn near him only when I'm there. This has gotten to a point where he can no longer get levels, or run his mob farm as things work only for me.  And yes I have tried lighting every cave system in hopes we would find a crazy amount of mobs. We aren't on peaceful and have no cheats running. Mob spawners do work for him though, which is strange