Test your seam allowance. Cut a 2.5" strip of fabric, cut that into 3 pieces, and sew those 3 pieces together. Press the seams to one side, then measure the width. If you don't get a width of 6.5", then the problem is with your seam allowance and/or your pressing. The seam allowance needs to be *scant* because when you iron the seam some of the fabric is used up by the "turn of the cloth". This is how cutting exactly and sewing an exact 1/4" seam allowance can cause your blocks to come out smaller than you expect.