Are you sure both sides of the roller are supposed to drive? The roller is in two pieces because when you turn corners, the two parts have to turn at different speeds. That means either only driving one side, or driving both sides but having over-running (sprag) clutches on both. If it has two clutches one of them might be stuck. Otherwise, it might only be intended to drive one side. Can you freewheel the driven side - that is, push the mower forward when the clutch is engaged? If you can, it has at least one over-running clutch.
