Loose stitches or knots on the top surface of the material

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.