I would be really surprised if it was a firing pin protrusion issue. Unless someone fooled with it and ground / filed it or replaced it, I can't see how the pin may wear. As stated above, check the inside of the bolt and clean it with a bronze brush and acetone. The protrusion can be checked, but usually, if there is a protrusion problem, it's much more likely to be "too much" protrusion than "not enough".
Also, and it's not common, if the rifle was stored for years with a "cocked" bolt, or if someone fooled with it, the spring may have lost some tension. Or the primers are very hard, which usually is not a problem with Mauser type actions.
As an aside, is there a tutorial somewhere on how to take those Model 4100 bolts apart please?