Probably your seams aren't all perfect. BTDT (and still do). You can adjust some seams to try to get it to fit. Sashing seams (the strips) are fairly easy to adjust-just sew the seam a tiny bit smaller or take them out and sew a bit bigger, depending on how you need it. I wouldn't even attempt to adjust seams in a pieced block unless it's a simple one like a 4 patch or 9 patch.