Small Appliance Repair Questions? Ask a Technician ASAP
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That might be either the bobbin being the wrong size (so, obviously, try a different bobbin or two), or the bobbin case being off-spec, either with poor manufacture or perhaps with being worn out.
Let me know whether trying a different bobbin produces different results. (Bobbins are not made as well as they used to be, not even by Singer. Classic sewing machine's parts were at least made with some considerable precision and quality control... but try a different bobbin or two, I'd say.)
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Might be the bobbin case, admittedly. But also check your tension settings. Knotting may be owing to loose tension; if the bobbin thread or bottom thread is what is knotting up, then the bobbin thread or lower thread tension may be too loose. Tensions might change, if a machine has been sitting for a long time.