You need to mark the sashing before attaching the next row. This is what you are missing.
Create your rows with short sashing strips between blocks. Attach long sashing strip to bottom of first row. Before attaching second row, turn first row over and, using a ruler and pencil, mark all the seams as if they were continuing into the sashing. When you pin the next row on, match all the seams in the 2nd row with all the markings in the 1st row. When you sew, ease as needed to keep those pinning points matched. This will ensure that all the blocks will be lined up and even when you are done.
An alternative method is to insert cornerstones (squares in the sashing where blocks join); however, that is a lot more work and changes the look of the sashing.