When I'm having a hard time with seams, I use Elmer's washable school glue. I take the pieces to my ironing board and place a tiny dot of glue in the seam allowance, making sure the seams match, and press with my iron. The iron sets the glue. Once the seams are all matched, I take it to my sewing machine and sew. If the glue needs to be loosened, a drop of water on my fingertip is enough to take the seam apart again.