I do recommend that you use your walking foot. I know some say to start at the middle with SID but I start at the ends and have no problem. I pre-wash all my fabrics and thread baste using Sharon Schambler's method but I don't think that should make a difference. I think it is your presser foot. I would continue with the rows you have not done with the walking foot and rip out the other ones while watching TV! Just another thought. I have not stitched diagonally until after I have done the horizontal and vertical rows. Could the distortion be because of the bias? Welcome to the board. You will learn so much. The people are wonderful here. So knowledgable!