Do you mean that the top thread is being pulled to the back of your quilt sandwich while machine piecing? If you have tightened the top tension and the top thread breaks, the problem may be the bobbin. Does your bobbin case have a tiny screw on it? Take a picture of the bobbin case and position of the screw for a reference. Place your bobbing case over a bowl (in case your screw falls out) and loosen the screw a tiny bit. Try a quilting sample and if necessary loosen again if needed.
If that is not the problem, refer to the picture to set the bobbin screw back to it's original position.