The restrictor plate was my guess as well. I've got an English IPA on tap that had a little bit of hop debris from dry hopping, and for a while the first part of the keg would start to kick out foamy beers, until finally stop flowing at all. Just unscrew the bottom of the tap and wash out the little holes, and see if that clears things up. Mine happened enough to cause a problem fairly quickly, if you only had a little clogging I could see it taking a while before it started to happen. Either way, its an easy thing to check.