The problem may not be with your machine. Fabric has a tendency to "drift" when you stitch it and curl in one direction. When you are sewing a strata for fabrics together which you will sub-cut into sections for squares it is always recommended that you sew one seam top to bottom and the next seam from the bottom to the top to keep your strata straight. It is necessary to guide your fabric through the machine to keep it feeding straight- a finger placed along the presser foot is a good way to do it. I like to extend the guide for making an accurate 1/4 inch seam to the back of the machine and guide the fabric all the way along the guide. If you don't keep the fabric straight it will veer off. It can be hard to detect in short distances but is very obvious in longer seams. Many people feel that the stitch on a FW is among the most perfect straight stitch because it only straight stitches. When you add the ability to zig-zag it is no longer possible to be absolutely straight and the tiny bit of zig-zag will pull fabric just a bit.