In some cases (unknown reasons) server stops spawning new monsters (tested only for monsters, but could be for all mob types). Currently I wrote mob cap balancer to limit monster spawning near the player if it exceeds limit + added ability to track monsters nearby. So I noticed the following situation, which can occur:
1. Limit of monsters set by plugin = players * 100 (and players * 70 for natural spawning).
2. We have two players on the server, located far from each other (they don't have shared chunks). Each of them has approximately ~70-100 monsters nearby (screen attached).
3. Then one of them log outs, the other walk so far it can load other chunks and gen other mobs. Now the monsters around him drops down to 0. Even if he logs out / logs in to server, nothing changes. Server reboot "fixes" that problem, also joining player could fix problem (monsters nearby spawns up to 70-100 near each player and again, on log out / walk drops down).

Same thing before was that I joined empty server and there was 0 monsters around me until I rebooted the server.

To make you sure that this is NOT plugin issue, the plugin does the following:

• On player join / quit changes world limit to playercount * 100 (/70).
• On monster spawn cancel it if monsters nearby player >= 100
  So it can't limit something under 70.

I hope some info useful here and will help to solve the issue. I'll be glad to provide more info if needed and possible for me.