I've seen a starter spin in the wrong direction. For some odd reason the "memory" of the starter changed and it did EXACTLY what you are describing. It would spin the starter, and the engine would stay still. This was a 97 GSXR 750 motor. The way I fixed it, was to disassemble the starter and pull the stator out of the starter (thereby removing it from the magnet, and "clearing the memory"), then reassembling the starter and giving it a short with the right polarity to "reset the memory". I know this sounds odd, that a starter could change it's rotation for some reason, and then be re-taught which way it should rotate...but it worked. Try it and give us feedback.
Ps. I believe the reason for the starter changing rotation, was that it had been sitting unused for an extended period of time.