Sometimes I think it is the weight of the material that pulls it rt or left away from the mark....that happens to me when I am putting a binding on a large quilt; sometimes its our own "nervous" hand motions, fussing, and of course we have all read the remarks that a FW sews a straight line....that is all it does. Well, with all those mentions it seems to me that a lot of people are trying hard otherwise to be "right on" the straight and narrow. I just go back and correct the deviations.