Did you make sure that the pin on the spindle fits into the slot in the bobbin?
Did you make sure the clasp/lever fits in between the sides of the bobbin and is touching the hub of the bobbin?
Sometimes the
pin on the spindle that kind of locks the bobbin
is worn. This makes sure the bobbin is being held to the spindle and rotates the same as the winder. Sometimes the bobbin's
notch is not deep enough to fit onto the pin.
A couple of threads that discuss 15 class bobbins can be found at I think there is another one that might be better, but I can't find it right now.
Singer 15-91 Bobbin Winding Issue
Bobbins
Janey - Neat people never make the exciting discoveries I do.