The hardest ones to open completely are the HST blocks. Those little ends seem to want to not open well. So I first set the seam, then, with light on the bottom, open the top and push the tip and side of the iron into the seam just a short way. Then I back off and push the tip of the iron into the seam at each corner. That way I know those corners are really pressed open. And pressing then from the right side helps, too.
Check each of the corners to be sure they are pressed all the way open.