I have the Starter out. It Spins the relay kicks in I guess it is in that clutch.
How are the splines on the armature of the starter? The splines on the inside of the SC? Those are two things that could cause you issues. Also, check the teeth on the drive portion of the starter clutch and see if they are making contact with the flywheel.
Has your starter been load tested? One rare failure on starters is that the stack seperates from the shaft on the armature resulting in the stack spinning but the armature shaft not moving. I've never seen that happen on a Mitsubia starter, but it is something to look at too.