Pouring hot, not boiling hot, but pretty darn hot water through the drain holes can help loosen organic material caught in there sometimes. A little Dawn in the water wouldn't hurt either. That is also a good thing to do every year or so to keep them flowing well. The rooter method Klaus described is about your only alternative. If I were having your problem, I'd do both, likely at the same time.

I also heard a story where a guy crushed up some denture cleaning tabs and blew the dust in there with a straw followed by hot water to successfully clean a clogged SR drain. This is total hearsay but if I had 'em laying around it's just the type thing I might try.