Mobs spawn rarely on server (0.6.1)

[~IceFox~]

It goes without question. The title of the thread says enough. I recently updated my server from 5.7 to 6.1 and it seemed, for a while at least, that everything was going just fine. After a few hours of playing I happened to notice (as did many of the players) that over the course of a few hours of gameplay they had only encountered maybe 2-5 individual mobs. A spider here, a zombie there, but nothing more than that. I myself saw 1 spider and 1 enderman for the entire 6 hours I was online. With one of the new mods requiring so many enderpearls it goes without saying that the hunt for endermen is sure to skyrocket. However, the lack there of makes this a HUGE issue for my players. I have constant complaints about the shortage of mobs and was wondering if there was someway to bypass this problem. maybe there's a bukkit plugin I can use or am I doing something else wrong?