Minecraft Junkie

Just want to preface this by saying I'm going to make some assumptions that could be wrong, so I'm sorry if this has been talked about before or I am wrong.

I've been playing on the server again for the last few days and have noticed that after the server restart there are almost zero hostile mob spawns (That don't come from mob spawners). I've run into this problem before and it due to the servers view distance. I think that the server currently has "view-distance=7" in the server properties file but for mobs to despawn properly (that's the problem - not that they won't spawn) you need to have a view distance of at least 10.

That brings me on to my second point of Spigot. The only real reason you would lower the view distance is because it causes more lag, loading and unloading that many chunks. Spigot is based on Bukkit and compatible with all Bukkit plugins but has a lot of improvements towards performance. It also has a very efficient anti x-ray built in. Just sayin' it might be worth looking into changing over, it's really simple and from my experiences the performance increase can be pretty big.

Links / Source:
http://www.spigotmc.org/wiki/about-spigot/
Used /version in game to see you were using craftbukkit

http://minecraft.gamepedia.com/Spawn

I've found personally that hostile mobs tend to spawn en mass around my estate. It may be just how many people are around you, or how many are on the server.

It does depend on how many people are online, but more specifically how long the server has been up and how much those players have moved around.

To be more specific of the problem, mobs instantly despawn when over 130 blocks away from the player, the problem being on a lower view distance server the chunks unload before the mobs despawn. (View distance 7 = 112 blocks). Those mobs even in unloaded chunks count towards the limit of hostile mobs that can be spawned. So people running around loading and unloading chunks fill the mob limit almost instantly.

However, people running around loading those chunks again and killing the mobs will often lower the amount of spawned mobs again allowing more to spawn. This is why when you are running around you will see zombies and skeletons stood in the middle of nowhere burning to death.

For you though, if you are only on your own or very few people online that are not moving around that much the mobs don't have a chance to get stuck inside the unloaded chunks making the spawn rate normal.