I posted this response to your other post with this same question:
"Was the PTO shaft from the tractor spinning, but the tiller was not? It can look like the tractor PTO is not spinning because the shield is not. Do you have a slip clutch on the tiller? More than likely, the shaft is spinning inside the shield, and the slip clutch on the tiller is slipping. Readjust that per instructions."
I think your slip clutch on your tiller is just too loose. Mine does the same if it's set up a little too light.