Zero Turn Mowers

I have a RZ4219 zero turn mower. The cut I'm getting from the right side blade is very poor...the left side cuts great, but the right side is so poor I have to re-mow it with the left side blade. I've had the blades sharpened, checked tire pressure and the mowing deck is level. It seems to me that the belt is looser than it should be. According to the manual, there is no way to adjust this (?). Is it possible the belt has stretched, or is there some other way to address this?
also the company i bought it from went out of business after 5 months,and they did some changes on this zero turn,for example changing pully size,telling me they were the wrong ones,bought brand new from dealer,and not very happy. please someone help..thank you..ron.b.