The 870 has a stellar reputation for reliability, but I have seen trouble with some, including a half dozen or so 12 ga "Magums" used as patrol guns. My ol'reliable has been burping a bit too.
The prob with all has been the carrier not raising the shell sufficiently to allow the bolt to push a live round at the correct angle into the chamber. I suppose correctly called a "failuure to feed". The solution for the patrol guns was a new "carrier dog spring". I have not addressed my old gun, but will by next spring turkey season.