I've owned the same Ford and AirSafe hitch setup for several years. Here's a few things to consider. I'll assume that there are no leaks since you couldn't find any.
Does the hitch physically top out (ignoring the scale) at 100lbs, loaded? Before you "recharge" back to 100 psi, how many psi does it lose? If your concerns are based solely on movement of the scale, you need to take friction into account as my hitch sometimes settles lower than it rides. If you back up and stop quickly, does the hitch settle to the desired height? Also, I visually check height by glancing at the amount of travel (drop) in all four corners instead of the color scale. It's something AirSafe suggested to me when I had a question for them on the subject.
Dry lube silicone spray in the rail area of all four corners resolves friction problems.