What about the nose (hitting part) of the firing pin/striker?
Is it deformed, can it be "rechaped/sharpened" without the danger of perforated cases?
Another thing is case-rim-thickness.
About 25 years ago a gunsmith in Holland offered a rimthickness gauge.
He said that rims vary so much within one lot/box that it payed to buy a gauge and sort them, within one lot, in order to get a more concistent point of impact.
Furthermore you need to check the "head space" i.e. the distance of the front of the slide against the breech. I 've had some FN internationals in the club that consistently missfired if dirt accumulated on the slide face/breech end of the barrel.
Apparently the slide consumed some kinetic energy of the hammer by being slammed forward at the moment of the hammer hitting the striker.
Hope it helps.