I have two grinders in the utopia wilderness, but both are not getting any spawns today. What can cause that? They are usually pretty steady.
Ok, you might want to check in the Aikar about this, his in-depth thread is here: http://empireminecraft.com/threads/attention-exp-point-grinder-users-read-texp-leaders.6665/ Other than what is mentioned in the above link, I cannot suggest much more. My guess is that someone's afk grinding and busting up the mob cap...