i have gammill clasic plus no zipper leads so can't answer that one but Are your birds nest where you start? Are you pulling the bobbin thread and hold both till do couple stitches to lock in place. Same thread top and bottom? I was told not to have needle in straight north__south more a little wonky like north-west to south-east this angle \\\ helps thread travel better and you can't adjust unless have a full bobbin to start with what do you have your stitch set at if yours has the regulator try lengthening so can see where messing up Sometimes find its computer thing Shut off..go have a cup of tea ..unthread all top and bobbin rethread turn on and try again sometimes its the little things bobbin going clockwise,clicking in, top thread slips behind tension instead of through it I'll keep watching this