If you don't have your F/L die screwed in far enough, it will not bump the case shoulder. Screw the die in until you just lightly touch the top of the shell holder, measure the fired case shoulder, note it down. Size a case. Measure the shoulder. If you're not far enough down, you may see a slight lengthening. Keep screwing the die down and keep measuring and screwing the die down until you start seeing the shoulder measurement getting shorter. You will find the sweet spot. You go in small increments like 1/12th of a turn or less. It's a fine adjustment when you hit it, just like Erik pointed out in the video.
For a gas gun, I bump .004"-.005".