Take a ruler and line it up against every other row. That should show you where you are off. What I would probably do is simply adjust and cut off whatever doesn't line up at the ends.
If you are duty-bound to take all the rows apart, you can likely avoid the issue by using a pencil and ruler on the wrong side of the sashing strips to mark where every block starts and ends. These become your "virtual" seamlines for lining up. Pin top row with sashing to next row, matching actual seam lines with virtual seamlines that were marked on the top sashing, then sew. Do the same for each subsequent row. You need to ease or stretch the seams when you sew in order to get the pins to match. If you don't do this, chances are when you sew all the rows together again you will still get uneven edges.