if it is primed, then i would suspect possibly a broken drive key that transfers torque from the drive shaft to the gear. Many units use a small plastic key, and a replacement can possibly be made from a on offcut of PVC pipe.

I had a  Macnaught chemical transfer pump that was very touchey about  how tight the screws holdng the face on were... any tighter than finger tight and the gears would drag and the key would shear.

One other thought you should check first... the polarity hasnt been revesed, and it is driving backwards..?