ltx1000

I have a LTX1000 that doesn't go in gear. I checked the belt and pulley and everything seems fine. What should I look for next?

I have a LTX1000 Craftsman Rider. I can lift rear and engage forward and reverse and wheels turn. When wheels are on ground they don't move.When I put in gear Transmission sort of growls but doesn't move. Belt seems fine. Any suggestions?