Ok, I've got a John Deere LX280 mower and I am having problems with the mower deck, specifically the belt that goes from the motor to the mower deck. The belt is new and the correct size. When the deck is in the up position the belt is tight, but when I lower the deck in the mowing position the belt loosens up. Because of the slack the blade gets in a bind in grass that is less than 3 inches high. I don't know where to go from here.