First are they cut accurately? Do you sew a series of 2 patches first? If so, sew the seam, then press and make sure the edges line up. If you start with them a little bit off, it will only get worse. If you can press the seams in opposite directions, that helps to nest them. Use pins or glue. Not sure what your pattern looks like, but I have better luck if I piece in smaller square chunks rather than long strips.