Ocracoker, I'm sorry your Shield didn't work out for you. You certainly gave it every opportunity to do so. Sometimes, we gotta do what we gotta do. Hope you enjoy your Kahr, and better yet, that it provides you the reliability you need in a CC weapon, whatever the brand.
Roll pins are funny critters. From many years of machine-shop experience, I know that they can be kinda particular about the size of the hole they're made to fit. The right size hole, they're golden and won't budge at all. That same hole, if it's a few thousandths too large, is problematical, even though roll pins are designed to expand (and, thus, to be tight in the hole they fit.)
If S&W, in the process of manufacturing thousands upon thousands of Shields, had a bit of "run-out" in the drills used for the roll-pin holes, some of the holes could have been enough out of tolerance that the roll pins would be loose enough to start working themselves out under the impact of rounds fired. Just an opinion, of course, but I think it is an informed one....
But it's not only aggravating, it's also a structural-integrity question, if a roll pin works its way out regularly. I don't blame you for not putting up with that.
Good shooting to you. Be safe, and again, hope your Kahr proves to be reliable and trouble-free.