Have you successfully been able to quilt on the frame before taking it in for repair?
If so, have you:
Rethreaded - completely
Rewound your bobbin
Changed your needle and perhaps type of needle
Oiled
Are you remembering to lower the foot when quilting
Try changing the thread to another spool - I had a problem and it turned out to be a bad spool of thread
Is your sandwich too tight/loose?
Are you going too fast? Are you slowing down at the curves and points?