Make sure your blocks are large enough before sewing. 1/4 wouldn't make it enough to cause it to be off 1/2 inch. Believe it or not; not all rulers are created equal either. Check the size of your blocks and then also sew a test strip to see what your seams are. Maybe your foot isn't accurate and it if isn't allowing you to move your needle over far enough then you'll have to use a different foot that will.