I can't remember how the race and hook moves on the 401, but if I remember correctly there's a tiny lever that relases the bobbin case and needs to click in place when it's put back. The bobbin case rest on the turning parts (race and hook) and it always helps oiling these. It may just need a bit of oil, but I guess some parts might need tightening up under there. I think some reccommend oiling the 401s every other week to keep them smooth.