Mobs won't spawn when 4 or more players

Long time ago we created one world and we were doing so great until suddently some day mobs stopped spawning (except if you were near to a spawner). It only happened when 4 or more players were playing. We thought it was due to some kind of bug related to new textures so we began a new world. And we're now experiencing the same problem. The nether is empty, no mobs at all of any kind. In the overworld, no animals or monsters generatie, we can spend 10 nights without sleeping and the phantoms won't show up. Then, when one player leaves and there is only 3 of us, phantoms and everything else suddenly appear. It is so frustrating.