[philso]

Both the AndyMark and VexPro are supported on the shaft by two hubs, one on each side of the plates/bodies. If the two hubs become misaligned to each other (axes not parallel or axes parallel but not coincident), they can jam on the shaft. You might try loosening the screws that hold the hubs onto the plates/bodies.

We have had similar problems getting shafts into the new clamping gearboxes and bearing blocks. The shaft is a slip fit onto the bearings leaving very little tolerance for misalignment between the two bearings.