I've had an occasional FTF with Remington Copper Solids in my 12 gauge USH. Slight dent to the primer but no ignition no matter how many times I dropped the hammer on it. Other brands that I've tried (Federal, Lightfield, Hornady) fire fine though. I'd say out of about 200 rounds through my USH since I've owned it, I've only had about 3 to 4 fail like this, all Remington. I can't say the gun is at fault so I suspect its just a hard primer or a deep seated primer. The ones that don't fire will always fire out of my Rem 870 so the ammo is okay.
My two cents.
Good shooting!