Are you “spining your seams “like Bonnie shows? I would try not finger pressing, just do that and put iron straight down on it. I press with seams up but I know some press the other way. If you do finger press use you nail not your finger pad. I do set the seams of each strip set before matching them up for the sub it. I also am using a pressing block on top of them right after they are ironed .
Hope this helps a little.
Terry