I have heard of this before with absolutely, 100%brand new, straight out of the box, 870 Express's. Remington sometimes leaves a bunch of gunk in the barrel after test firing it at the factory.

Simple solution: Use CLP "Break Free" and a bore brush and scrub the $*&# out of it. Then follow with some patches and finally a tiny dab of gun oil.

99.999999% of 870's are trouble free and are the #1 most reliable, most popular, shotgun in the world.