As others have said I think the problem is the short stitch length. I have the Crescendo and it has not given me any problems sewing over thick seams. I hemmed some jeans a couple of months back and my Crescendo sewed through the thick seams like butta baby.
Maybe you can trying sewing with the shorter stitch length on the part that doesn't give you problems and then lengthen the stitch when you get to the thick part.