Are you pre-measuring and cutting your sashing strips? You really need to do that; otherwise quilts tend to "grow" in unwanted ways.
Another question: have you pinned two rows together without sashing just to make sure that your rows are of equal lengths?
Also, are you marking the wrong side of your sashing before sewing two rows together? If not, that should help. After adding sashing to the bottom of the first row, turn it over and (using a pencil and ruler) mark the seam lines into the sashing strip. Pin the next row to that sashing strip before sewing, matching the seams on the 2nd row with the lines you have marked on the sashing. If the sashing is a little full, ease the sashing to match the new row. If a block in the new row has a little excess fabric, ease that to the sashing. Doing this will ensure that your blocks are all lined up with each other.
If you do this without pre-measuring and pre-cutting the long sashing strip, I think you will find that you have excess sashing to ease in on every block.