If its a rider then it should have just belts. Check the belts for where by just tipping the mower on its side. Send us some info on what make and model it is too
How thick is the grass? Check the cable adjustment and be sure its giving its MAX throw when you engage the handle. if the grass is not 12" high and everything else is good then it may be your clutch system. Also how old is it? it may just need some maintanence done to it.