This is a pretty common issue with the 870 express. The rough chamber causes all sorts of ejection failures and/or rough cycling of the action.

A bronze brush with some 0000 steel wool wrapped around it, chucked into a drill and lubed with a decent oil will do the trick.

Yeah, I know, a new Remington shouldn't have to have this done to it but once that is done, it is all the problems you will probably ever have with it.