Sometimes it makes a difference what thread you are using whether you get a good 1/4" seam or not. The heavier the thread, the more space it takes up on the fabric from both pieces and your block will end up shy of the desired size. The needle size van also affect it. If you are using a 40 or 50 wt thread on top, you definitely want to move your needle to the right just one notch and check your seam, adjusting more if necessary. If your foot or throat plate do not have a wide slot in them, be very careful when bringing your needle to a new position.