Disassemble the offending bolt, and examine the tip of the firing pin under magnification. If it has pierced multiple primers, it's probably been damaged by leaking gas. The tip probably has a burred or rough area on it now, which will only make the problem worse. If it is damaged you may be able to repair it, but the smart money is on replacing it. Also examine your bolt face and firing pin hole under magnification. If the firing pin hole looks wallowed out in one area, it's may also need repair.