Take the free hub out and check the circlip/pawl spring is still in tact and the teeth/ratchets (or whatever they're called) are still in the correct place and check for any debris. Put it back and spin the cassette and see if you can tighten a little less than it was, or perhaps apply some grease. You can overtighten the hub I think. That circlip has snapped on mine. It stopped working for me until I put the ratchet things back properly and installed the hub correctly (which took me, a clueless IT guy, three goes lol).
