I use washable glue to match seams then ease in the rest of the two blocks to fit. The important thing is to find out why the blocks are off. Either your seam allowance isn't accurate, bad pressing, or fabric cutting is off. Even a smidge off on all pieces will make a mess of your piecing. If you can, put the bad fitting block on the end of a row. The least place a judge looks is the top left second row.