The lower thread lies straight on the underside of the material and there is the appearance of loops or small knots.
Possible Causes and Solutions:
Cause: The tension of the upper thread is too weak or the tension of the lower thread is too tight.
Solution: Tighten the tension of the upper thread, or loosen the tension of the lower thread, until the tensions of both threads are correctly balanced.
Cause: The machine is not threaded correctly.
Solution: Re-thread the machine correctly according to the instruction manual or video.
Cause: The thread is caught or stuck at the thread spool
Solution: Verify that the thread is not caught or stuck at the thread spool, thread stand or thread guides.
Cause: The bobbin base is not threaded correctly. The bobbin unwinds itself in the wrong direction.
Solution: Thread the bobbin base correctly.
Cause: The point of the needle is bent.
Solution: Replace the defective needle with a new one.
Cause: The bobbin is not evenly wound. It unwinds itself irregularly.
Solution: Replace the bobbin with one that is wound evenly.
Cause: Lint or dirt in the bobbin base prevents the bobbin from revolving freely.
Solution: With a pointed pin remove all dirt from this part. Clean with kerosene and dry thoroughly before replacing in the machine.
Cause: The thread take-up spring (check spring) is not adjusted and does not work properly.
Solution: Adjust the thread take-up spring until it has the correct tension and its upward motion is just finished at the moment the needle enters the material.