With 10 million 870s out there, my guess is 9,999,999 or so of them are still out there working well and sans glitches.
If takes serious abuse and neglect to make an 870 NOT work.
A couple areas to check are....
The extractor should be unchipped, with a good hook showing. Look at one known to work as an example. A new extractor is inexpensive and easy to replace, I'm told.
The Ejector mounted on the receiver wall sometimes gets chipped on the forward corner. Feel for roughness, if none you're fine. A new ejector is a factory replacement.
Wear marks on the mag tube are simply indications the 870 has been pre-smoothed.
HTH....