Zozee, if any of your pieces were cut at 4 1/4" rather than 4 1/2 I think it will not match up. The only other thing I can think of is to be sure your seam allowance is only 1/4"-- if you sew it wider the 4 piece block will be smaller than the final 12 1/2" piece sewn on the end. Maybe you should recut just a new sample of 5 block pieces being sure to cut them at 4 1/2" and assemble them to see if it comes out square. And be sure your starting square in the background fabric is 4 1/2" square on all sides.