What could be wrong with my lawn mower?

I have a 3 year old Craftsman self-propelled mower. No matter how long the gr is when I mow, or how high or low I have the deck set at, I still get a bunch of clumps of cut gr. Often times in a long line along the path of the wheels. This is the first year I have had this problem. I got the blade sharpened and the problem still happens. And, yes, I have verified the blade is not on backwards. Help!