Have you checked to make sure the bar that needs to be over the screw (I have a Bernina so I don't know if it is this way on all machines) is attached just right. I am speaking from experience--I sewed something over the weekend with the walking foot and also ripped out and then realized that the bar wasn't exactly on the screw and it didn't sew evenly at all. Took me a second to readjust it and then it sewed beautifully.