Okay, so. I am sewing with a Singer Touch & Sew, model 604, in pretty good condition. I have been using it for a week or two now to sew underpinnings for an exhibition of historic fashion - the materials are old cotton sheets and polyester sew-through boning.

Everything was fine until the last time I refilled the bobbin. (For reference, with the Touch & Sew the bobbin stays in its case, you just press a button to put the machine in bobbin winding mode and hold the pedal. It's magic.) Ever since then, when I try to sew the top thread breaks after a stitch. I've oiled everything up, changed the needle, tried different tensions, etc.

It's the bobbin, isn't it? I know it has to be the bobbin. Do I just cut all the thread off and rewind it? Or is there something else I'm missing?

Update: I did cut all the thread off the bobbin and rewound it, and it is still happening.