I have been playing on a vanilla server, alone, I do this to avoid changing the deficulty settings, using default server settings and I noticed that mob spawning seemed really odd.

I have a Hostile Mob less Enderman farm and it causes mobs to drop 22 blocks from 127 down to 105 which is my xp killing floor level. The highest terrain in the area is 76, so I have made an idling platform at 204 or 128 blocks higher that this and only 100 higher than the mob drop zone. Now here is where the minecraft issue comes into play.

As I mentioned above, I play solo on this server, and my spawn rates were okay if I constantly went up and back down, but as soon as I idled at the top floor, my rates for spawning dropped to 0 in the mob farmm which the bottom floor was only 80 blocks away. I decided to switch my world over to the saves folder and play it single player, and instantly my mob spawn rates are back and they still work while I idle on my idle floor.

There seems to be an issue with spawn rates or distances on servers.