I also tend to have the same problem and it happens when I even use a guide..I found that I have a tendency to let my finger stay on the fabric and my finger bumps up against the foot and it actually moves the fabric away from the needle..I need to keep my finger more to the left side of the fabric away from the presserfoot..but at the same time perpendicular to the guide as it's sewing...If your not using a guide it might be that the fabric is being shifted because you don't realize you're pushing it over as it's being fed to the needle...it might also be that you're letting go of the fabric to soon and the feed dogs just take...Hope what I don't do right might help you...