Bronica SQ film not advancing

My bronica SQ B is no longer advancing the film in one of my backs. The spindle at the bottom (take up reel) turns but the number does not advance and there is clearly no film moving through. I can't quite figure out how the spindle can turn without advancing the film but that appears to be what is happening. I fired off 12 shots and pulled the slide to see if I could see the film and it was still there.

Help! Help?

Does it have a Multi exposure switch like a SQA? If so, has it been accidentally switched on?
Alex

This might help, the first post is for ERTS backs and a later post for SQ backs: http://www.filmwasters.com/forum/index.php?topic=6470.0