It looks like your blocks may have some bias edges which can cause them to stretch. Take it apart, starch and iron your blocks, check to make sure they all measure the same before trying to re-sash. When doing sashes you need to pin well and ease in any extra fabric so that the blocks all line up with the pre-determined length of the long sashing pieces. When ripping out long stretches of sewing, I like to cut one side of the bobbin sewing thread (use a ripper or pointed snips) every 3 to 4 stitches. This allows me to pull the whole top thread and it will come right apart.