I find that often the problem in accurate piecing is the first 1/2" and the final 1/2" or so inch on the seam is a problem. Slow down and watch these areas to be sure your seam remains consistent. Also......are you pressing or "ironing" Ironing can stretch. Pressing is up and down with the iron. Care is essential. Also when piecing triangle shapes, be sure when they are joined to other shapes you have the 1/4" at the ends.,