Your seams are likely a little too wide. That makes your blocks short for the length of the sashing strips. I would measure your blocks and adjust the cut-length of the sashing strips to match the unfinished length of the blocks.
Actually, if you have already sewed the sashing strips to the squares, all you have to do is remove one seam (say, on the left side of each square) and re-sew to make the sashing length match the blocks. I would re-sew first and make sure everything is accurate before trimming the extra sashing from the seam.
I am assuming that you have assembled the quilt in rows, with the blocks in one row and the sashing strips in a separate row.
Hope this is clearer than mud!