Lesco Truckster Agitator Bushing Question/Problem

I've got a Lesco Truckster, newer model with the top mounted motor. My problem is the nylon bushing where the motor shaft passes through the hopper on its way down to the impeller, will not stay down in its hole. After the motor has been on a short time the bushing seems to work its way up the shaft, which in turn lets a small amount of fertilizer pass through its hole in the bottom of the hopper.

If the bushing is out, the shaft to the eye appears to have some wobble when turning. If...

Lesco Truckster Agitator Bushing Question/Problem