I have plastic bobbins for one of my machines. When my stitching creates loops, either top or bottom, I have been able to correct it by using a new plastic bobbin. I have been told by the Brother repairman that the plastic bobbins warp. Also, make sure you are using the correct product number for your bobbin, the one your manual specifies. It does make a difference for success. Please be sure to publish your "fix".