When the grass bag is full, and I try to run the lawn mower up the hill of our lawn, the self-propelled system does not seem to have enough torque to drive the wheels. Hill is approx. 30-40 degree incline. Wheels do not spin or slide, they just will not turn. Is t here a way to increase the belt tension to increase the wheel torque. We just got the lawn mower yesterday and used it for the first time, so the drive belt is brand new, and will only get worse with time and use.