I've had that happen with several sxs guns & one Merkel O/U.

If you haven't already done this, first try a different primer if reloading or a different brand factory load. I've had more problems with Winchester 209 primers than any other & sometimes other brands will work w/o any problem.

Polishing & or slightly reshaping the firing pin tips will usually help but it's a trial & error process & use care not to remove very much material.