A few weeks ago, my husband was making and testing some 9mm rounds for my son and I to use. He would make a batch, then test it in each of our guns (my M&P and my son's Sig P226). Each time, he was having a great deal of difficulting getting the rounds to put the gun into slide lock when the magazine was empty. He tried several different formulas, thinking that they weren't powerful enough, tried crimping them down better to increase pressure, and still, had the same problem. The guns just wouldn't slide lock. Finally, he came in looking pretty sheepish. "I figured out the problem," he said.
"Really? What is it?"
"Well, my thumbs are too big."
"What? Your thumbs are the same size they've always been!"
"Well, I'm putting too much pressure on the slide-lock release when I am shooting, so the guns just won't go into slide-lock."
Now, clearly that is the opposite problem from what your friend is having, and maybe it really is a problem with the gun, but it wouldn't hurt to have someone else shoot it and see if they have the same kinds of problems. Maybe even someone with less experience, or a different grip, or who has a different type of job or something like that. When you are really good at something it can be difficult to "think outside the box" on issues like these.