Are you sure your seams are 1/4 on all sides? When I was having more trouble with accurate seams this was more of usually the problem. I also make sure to square every segment of the block as I assemble. If the segment is a 4.5 inch block, I make sure to find the exact center at 2.25 inches and line up my 45 degree angle with seams or corners. I know it sounds like a pain in the tush but the final block is usually right on the money. Oh, and press each seam.