Hummm...something is preventing the fabric from being moved backward I would think. Have you tried using a different foot to see if it makes a difference? There could be a scratch or burr or something on the bottom of the foot. Feed dogs will wear out after a lot of use. How old is the machine? Did you take the stitch plate off and clean all the lint out from underneath, not just remove the bobbin? I would also rethread the machine completely as well as try a different bobbin. that is just a basic thought for any stitch problems.