Problem with allocated RAM

When my server was one-player mobs spawned fine, but now that it's a two-player server the multiplayer-chunk-cache has a max of below 200, or a max render distance of 7-8. This makes it nearly impossible for mobs to spawn, which breaks my server's mechanics. I researched the problem to see if it was my fault somehow and everything says that this means the server does not have enough RAM allocated to it.

Please fix this,
LJ Talbot

It's not an issue with my grinders, there are other people on the server too... but some of the mechanics the server has involved mobs being stronger than normal, and neither me nor anyone else who plays on the server (I asked) is one of those plebs who don't want mobs in Minecraft. So since absolutely no mobs are spawning, I'm gonna repost this in another place.

EDIT: What kind of mob grinder works without any mobs?

I put monster spawning on hard, and there are no plugins (vanilla) to mess stuff up. So like I said it's not my fault, posted in the wrong area, should've posting in the feedback/suggestions area. Thanks for your troubleshooting but I already have checked everything you mention.

Also, it wouldn't matter what the mob settings are on (yes mobs are enabled on the server page) if the max render distance is 7-8, mob spawn in render distance 8-9. So I have one mob every few hours (using F3 to check if mobs are spawned, not just by what I see)