I'm assuming you mean that your block size was 12 inches finished? If so, 18.25 is the correct size to cut your squares for the setting triangles. Is it possible that you stretched the bias edges when sewing the triangles? If so the best solution is probably to take them off, starch the fabric heavily, press it carefully to the right shape, measure and trim if necessary, then re-attach. Maybe there's an easier way - a picture might help.