OK, the quick and easy first:
Remove and dis-assemble bolt, which contains the firing pin.  Clean thoroughly, inspect for interference of firing fin travel and erosion of firing pin tip.  Replace/repair as required.
Replace hammer spring.  Inspect hammer for interferecne in its travel, including interference by bolt.

If that doesn't work, try this:
Inspect barrel extension where cartridge seats.
Excessive dry-firing will depress the area struck by the firing pin, thus removing support for the cartridge rim just where it is needed for proper ignition.
Brownell's makes a swaging tool that might swage out the indentation made by the firing pin.  The other alternative is a new barrel.
Hope this helps.
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