Engine pully turning. Transmission not slipping and shifting arm is properly engaging. Linkage and belt loose with clutch on or off. This is with tractor off and proped up.

Hi,
Always wise to give a brand name at least. If it is an MTD Tranmatic, try putting it in the highest speed and releasing the clutch to unstick the variable speed pulley. If another brand, suspect a defective idler on the clutch mechanism.
Peace,
Clarke