Recently while I was teaching a class, a student was having all kinds of problems with her thread breaking. We tried so many solutions and none helped. Finally she took apart the whole bobbin assembly and finally found the culprit: a 1" scrap of pink thread that had gotten jammed in the bobbin race. Once she got it out, the machine performed perfectly.
Keep in mind that there are two types of bobbin race -- drop-in (shown below) and front-loading -- so the process of finding that little bit of thread will be different on each type. But now you know what to look for . . . .
Who knew something so small could create such big problems?!
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