Sounds to me two things are happening, 1 the top aluminium threaded coupler is stripped or 2 the bottom part of the fork has stretched past the captive weld part of the fork. Remove your wheel and pull on each fork one at a time. if one pulls out you have a serious problem, If both come out its really bad. finding springs or the top coupler are pretty hard, so I would shop around for a decent used fork or just get a new fork set up all together. Hope this helps!!!