Have you tried shortening the stitch a bit? Or turning the wheel by hand through those thick seam areas? You said you did try a bigger needle which should help prevent the needle flexing. umm, I have had the quilt hit the wall behind my machine and prevent the quilt from moving forward smoothly. I used to have my mother help me keep the quilt moving smoothly as it is so easy for the weight of the quilt to keep the quilt from moving easily and evenly.