I had the same problem the one time I tried doing the sashing and blocks in long rows. If you are off a little in seam allowance, you tend to be off A LOT by the end of the row.
After that, I tried adding the sashing and small block to the big blocks individually (like to the side and top of a block) and then attached the big blocks to each other. I had much better luck getting the seams to match up!