I make and use a lot of 9 patches and I have learned that most of my non-matching problems come with the pressing. I seem to be prone to pressing in a lot of little extra folds on my seam lines.
I used to always press from the back first, then from the top, and that is where so many of my pressing errors occurred. Now I press the front first, making sure there are no tucks or folds at my seam lines and my accuracy in matching squares and seams has improved at least 75%.