Problem with NEF

Rough or rusted chamber, clean and polish.
Grease or crud in extractor groove, clean and lube.
Weak extractor spring, replace.
Reloads? Too hot? Back off.
Dirty or corroded brass. Obvious.
Didn't specify if rifle or shotgun. If shotgun, probably steel base promo shells.
That enough to start with?

Probably crud in the chamber. Fired she'l expands getting a tighter grip.

Crud in the chamber or anywhere else you'd be able to see. Bath first though. Kind of it'll eject a loaded round but not an empty.
I'd be guessing the ejector spring may be broken or full of crud assorted. The extractor and ejector are the same part and may be rusted if the thing has been living in your jeep. Fortunately, despite that NEF has been absorbed by Remington when they bought Marlin, Gunparts has 12 ga. extractor/ejectors at $7.65 and the spring at $2.70. Listed under New England Firearms Pardner.

I had the same thing happen to me with a NEF handy rifle. Sent it back, and the problem persisted. Got rid of the rifle.

Your ammo could very well be the problem. I've have Winchester cheap junk ammo that would not eject from my Beretta 424, changed ammo, no more problem. Seem like all of the major ammo companies are making some cheap ammo that is real crap. If it's in a bulk box at a great price there probably is a reason. Cheap is not always a good deal.

Indeed, some very low end factory shotshells seem to crop up in stuck shell or weak ejection threads quite regularly. Try some different loads before assuming your shotgun is the problem.