I tend to respectfully disagree with part of Uncle Matt's assessment. I do not question his comment about the Remington slug ammo. That may indeed be the case.

However, you stated that experienced "mis-fire" issues with two different makes of ammo ... Remington and Federal. You also stated that on one case, the firing pin did not strike the primer (Federal ammo) at all. These are big red flags that point me towards the fire arm and away form the ammo ... how ever I certainly respect Uncle Matt's experience with the Remington slug ammo.

First I'd disassemble the bolt and check for debris that might be restricting the firing pin's motion, and carefully check for a cracked or broken firing pin spring. If there is no glaring reason that the firing pin should not work as designed ... once cleaned, lubed and reassembled, I'd head to the range and try again. Take some Remington, Federal, Wichester, etc. loads. If you get a similar result ... go straight to Mossberg or a reputable gun smith. It is the fire arm.