When I have had this problem, it seems to be always related to how the quilt is draped around the machine. The cure for me would be to get one of those suspender setups that lift the quilt up above the machine. (I will post a photo if I can find it.) Any type of drag on the quilt will cause the kind of problem you are describing -- e.g., if the quilt is hanging down over the front edge of the sewing machine cabinet, over the left side edge, or over the back edge of the sewing machine cabinet (or machine).
Edit: I found the website of the suspender setup another QB member posted about
:
http://www.jennoop.com/suspenders.html
She really likes hers, but I haven't been able to find that thread.