I'm just wondering if it's a thread size issue. Maybe thread and needle size aren't matched up? Which thread do you use? As Phillip suggested, make sure ([https://www.mybluprint.com/article/consider-this-your-go-to-guide-for-threading-a-sewing-machine|here]’s a nice tutorial) that you have threaded the spool correctly. [br]
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I would clean everything including the bobbin tension plate on the bobbin case. [br]
I would oil the machine especially at the shuttle race. [br]
I would take a fresh needle. [br]
I would take the presser foot lever up and carefully thread the machine completely new and look how the take up lever handles the thread. [br]
I would fasten the two thread ends when I start to sew. [br]
I would watch the spool. [br]
I would use a cone stand if the spools are to big and I would with certainty also check the bobbin tension. [br]
If the bobbin tension is too strong for your new thread than the top tension can be much too weak. (A small test - try to put the cotton thread in your bobbin, take the polyester thread as top thread and watch what happens). [br]
If I would get then a good result I would use a different bobbin thread color, sew with a medium zigzag stitch and look at the stitch line whether I can see the bobbin thread on the top or the top thread underneath the fabric. [br]
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If all that doesn't help (and if I were you) I would try it with a high quality polyester thread (but keep in mind, that some of the [https://sewingland.org/best-embroidery-thread/|threads] out there aren't made for a home sewing machine!) and start again to find the reason. It can sometimes be very tricky.