If you are certain a new firing pin is in the gun. I would check the sear. The ultimate way to determine where to start would be to check the headspace. But a quality 12 ga headspace gauge would cost more than the gun is worth after you throw in the cost of parts. If you have a regular gunsmith he might check the headspace as a charity case.
Another way that might get you started in the right direction is to put a plastigauge on the area where the bolt meets the barrel. If the bolt is not snugging up the plastigauge will not be flat. You can get a plastigauge at any parts house for around $1.