I lengthen the stitches also and it sounds like the weight of your quilt may be pulling and distort the stitches. I do mostly SID (stitch in the ditch) and set up the ironing board behind my table so the quilt won't pull off the table as I work it. I also hold the roll up as I guide it through the machine so the machine doesn't have to labor as much. It is hell on the shoulders but works for me.