My 870 was having the same problem and I ended up buying a new gun. The new gun seldom if ever had problems because it had a double lipped extractor. But the real issue was steel components on my shotgun shells getting wet and rusting just enough to become stuck in the chamber. Two things. I never let my shotgun shells get wet without wiping them down and drying them out. Rusty one get thrown away. Also. Took fine grit sand paper to the inside of the chamber and put some grafite on it. Now the 870 works without issues but I love my other guns more. Longer barrels. But it’s almost surely the wet shotgun shells.