You need to loosen the screw on the bobbin case, and tighten the tension on the top. Your top thread isn't tight enough to pull the bobbin thread toward the top and make the stitch properly.

Try changing your needle to a ball point, too.

Kathieo - how did you make out? Just curious.

Well it was an easy fix in the end. Before I heard from evaf I started with the simplest....rethreading the machine. I already had a ball point needle in so I knew that wasn't the problem. I cleaned the tension discs and rethreaded and that solved the problem. Working like a charm now.

Thanks for asking.