We tested our SuperBOBs M style prewound bobbins on over 100 machines. 98% loved them; 2% didn't. With further testing and research, we have a solution for the 2%.
1. If the bobbin tension is too loose, the factory preset setting may not apply the necessary brakes to stop it when you stop sewing. In this case, the bobbin thread continues to unwind, causing backlash, and upon start-up again, the thread will break. Tightening the tension will fix this. Think of a clock as you turn the large screw on the bobbin case, turning the screw equivalent to a 5 or 10 minute movement. Right is tight. Left is loose.
2. If the bobbin tension is too tight, remove the thin, plate-like insert in the bottom of the bobbin case. This is called the bobbin brake or the anti-backlash spring. It is easily removed by lifting it out with a pair of tweezers. This will reduce the excess tension.