Did you use the same ruler when you cut all your pieces? When I first started quilting, I learned that there are slight variations from one ruler to another and if you are switching back and forth, it could cause your pieces to be off.
Are you using a scant 1/4" or a full 1/4" seam? Are your seam allowances consistent throughout?
Measure after every seam you sew to make sure your block is coming out correctly. You may be able to find out where you are "off". Make adjustments as you go. You may need to tweak your seam allowance.