It doesn't seem to matter how accurate I think I am. If I'm ironing the seam allownance to the dark side, it throws the size off. If the fabrics aren't the same weight it's a problem. Sometimes the pattern isn't on the straight of grain so the patch has some bias to it when I fussy cut it and that also throws the size off. Starching doesn't seem to help either. I just have a block that is stiff and the wrong size as well.
I've finally have made peace with myself and acknowledge that I am not a perfect quilter and that's ok. If you are planning to compete with your quilt...that's a different story. You might see if you can get an experienced quilter to watch what you are doing and perhaps spot the problem for you.