Check the multiple exposure lever is actually turning the spindle it is attached to. I had a similar problem a couple of months ago. On examination I noticed a hairline crack in the plastic of the lever which was enough to loosen it and stop it turning the spindle. A little super glue used extremely carefully and it was fixed. Of course that lever can now not be removed from the camera, but at least I got the functionality back. (If this is what is happening with your camera, make sure spindle and lever are properly aligned before going mad with the glue!)