The upper thread lies straight on the top surface of the fabric and the lower thread appears there in form of small knots.
Possible Causes and Solutions:
Cause: The tension of the upper thread is too tight or the tension of the lower thread is too weak.
Solution: Loosen the tension of the upper thread or tighten the tension of the lower thread, until the tensions of both threads are correctly balanced.
Cause: The Tension Spring of the Bobbin Case is bent out of its proper shape. This condition does not provide sufficient tension of the lower thread.
Solution: Try to correct the deformed spring by carefully shaping it with a small round-nosed pliers. If not successful, replace the defective spring with a new one.
Cause: Lint, dirt, or pieces of thread have accumulated underneath the tension spring of the bobbin case.
Solution: Remove the tension spring. Clean the spring and the bobbin case, then replace the spring and adjust it correctly.