I use the plastic bobbins from Sew-Classic and mine often does not stay engaged either. What I have been doing is just holding my finger against the bobbin until it has completed winding. I tend to think the notch in the bobbin is not quite deep enough and that is why it slips, or it could be the thing on the winder that goes in the notch is worn down. I have tried some of the metal bobbins and have not been pleased w/them so I would be interested to know where you got your bobbins and how they do.