You can have a perfect quarter inch seam and have your block still not come out the right size. It actually isn't about what's on the seam allowance side of the seam. It's what's left on the other side of the seam that matters. Take three pieces of fabric 2 inches wide and sew them together. Measure the center piece on the right side and if it's not 1.5 inches adjust your seam allowance accordingly.