There are many may causes for this problem. The following is a list of
possible causes and fixes:
Bobbin tension too loose.
Tighten bobbin case (also see Setting Bobbin Tension).
Upper thread tension is too tight.
Loosen upper thread.
Clean bobbin case.
Bobbin tension spring is missing or weak.
Install, replace, or bend spring slightly to restore tension.
Use correct backing.
Scratches on rotary hook.
Polish with crocus cloth of #52c crocus cord if scratches are minor.
Replace if scratches are severe or rotary hook is scored.