I have a 988115 Pro G 48", 15hp Kaw. The mower cuts great and I have just 21 hours on it. The drive belts are already stretched to the maximum adjustment. I use the 2 wheel sulkie that gravely also sells. Is there any way to fix this issue?? I have adjusted to the specs in the manual but the unit wouldn't move period, with the maximum adjustment it is sluggish.