My RB kittie, Willie (also my first) had that problem. That is when the vet told me to watch the ash content of his food. He recommended Science Diet, and that is now the only brand of food all my guys eat, and no one has ever had a problem.

They also told me the less expensive brands have a high content of ash. At the time being a first time "mother" I didn't know the difference and just bought whatever was on sale. Boy did I learn my lesson fast!

Right now Fluffy and Yoshi are on that new S.D Adult Light/Hairball diet and its been working!! No more hairballs!! (gosh, I hope I'm not jinxing myself by saying that )