From what you describe.. It's possible that the top washer that butts up against the stopper ( those nipped bits on the main shaft ), may have slipped up and over the stopper. This would allow the spring to ride up too. So check that that washer is a snug fit against the stopper.. This would have been obvious when you disassembled it (If you had known what to look for at that time).
All I can think of...
Cheers
John