It is an inertial firing pin, so it travels further forward

by Catoosa, Saturday, August 10, 2013, 21:47 (4067 days ago) @ John K.

I'll bet John K nailed it. Unless the firing pin stop (retainer) fits tightly (seldom the case on a well-used gun) it will slip down when the pin goes all the way forward. If the upside-down trick works, try raising a burr on the inside (front) face of the retainer with a small prick punch. That will work for a while, but you may need to get an oversized f/p stop fitted.

BTW, a loose firing pin retainer will cause extraction problems, too, by allowing the extractor to "clock" or twist in its channel. Don't ask me how I know.