After having Kel-Tec twice sending me the newest assembly pin, because I've always had a problem with mine backing out (very rarely) and then telling me to replace the spring, I looked it over and decided that the spring I have seems to have plenty of tension, but the pin has a round radius where the low spot that the spring rests in, meets the high spot and needs to be slightly modified. I put the pin in a drill that I mounted in a vise and using the edge of a 6 inch flat file, which just filled up the low spot space, lightly squared off the radius. This was done under a lighted magnifier, but probably wasn't needed. I cold blued and oiled the pin and now when I put it in and remove it, there is a noticable "snap" to it and I believe that solved the problem. I did not make the part where the spring rests any smaller in diameter, but would have if needed, but didn't need to do so.
I picked up a used pre-owned P-3AT that has a similar problem. Finally, I got it under a strong light and noticed the assembly pin hole on the far side was wallowed out enough that the pin was allowed sufficient movement it would work it’s self out. Unfortunately I had to send the P-3AT in to have the frame replaced.
What gets me is the pins they sent me, were the latest ones and they still can't get it right. They just needed to cut a little sharper radius. It seems to hold a lot better now, but not enough to be too hard to remove.
The pin is a tight fit in both holes. I bought the gun new and always kept an eye on the pin, watching it after just about every shot. because I'd heard about the problem. As soon as it poped out a little, I put it back in. It's only happed twice in about 500 rounds.
This was more common with the 1st gen guns. The assembly pin was redesigned at least once. People now complain that the 2nd gen pins are harder to remove. I guess this is why. Is yours a 1st or 2nd gen gun? If it is a 2nd gen, I believe this is the first time I have seen this issue in a 2nd gen pistol. Perhaps your spring was sprung. :-/
Mine is a first generation 3AT, and I was aware of all the problems they had before I bought it, but I've been lucky. Of course I did the rampectomy and fluff and buff, so it is very dependable. Now I have the pin problem solved too.