Yes, once I removed the belt, I could see the spindle that the belt sits on moving.
So - lift the platter, nothing's moving. Remove the belt, see the spindle moving. Put the belt back on, spindle, belt, and inner platter are all moving. Put the outer platter back on and it's playing.
I don't know if something is holding the inner platform from turning? I don't understand why removing the belt and then putting it back on makes it turn.