Boots is right. Sometimes it's simple. Half the time I'll find a twig or part of a bush growing in there. Then you have other reasons. Sometimes there's a part broken or missing. It could be a pressure problem. Maybe a leak in a pipe you're unaware of. Sometimes they just stop turning and only God knows why. Sometimes putting in a new one makes everyone happy. You can just unscrew the guts and switch them out if digging it up is a problem. I've seen those cemented in a patio. I clean out the cannister with a garden hose before taking the guts out.
I just want to add that you don't want rotors and impacts running on the same zone. Either go all impacts or all rotors. Here's a link that will give you more info than you want to know. It might help you though. Good luck!
http://www.rainbird.com/pdf/turf/Impact_Guide.pdf