Out of time. my bobbin case keeps poping off the spindle. Once in awhile it allows the needle to

Question

my bobbin case keeps poping off the spindle. Once in awhile it allows the needle to hit the case which knocks the machine out of time. I have had it to two different shops. They tell me it won't do that when they have it. I have purchased new bobbin cases. I have tied winding my own bobbins. I need to know how to retime this machine myself as I can't have it in the shop every other week.