It initially sews correctly, and then the bobbin thread will snag, bunch up, and eventually break
There could be burr or nick somewhere that the bobbin thread is catching. Use a fine piece of fabric, like an old pair of hosiery, wrap it around your finger and run your finger around in the bobbin area and the needle plate. If there is a burr or nick, the fabric will catch. If it is small, it can frequently be buffed with very fine steel wool or fine emery board. Otherwise, that part can probably be replaced.
If you can, open the bobbin door and hand rotate the handwheel while watching the needle/hook action. You may see what is happening, ie the bobbin thread could be wrapping around something and hanging up.
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Increase the needle thread tension up a bit as it is too loose to pull the bobbin thread up properly
keep adjusting until both threads are at the middle of the materials being sewn
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