Luck of the draw
My buddy and I both own 870s in their Express trim. Mine is just the garden variety 870, his is the Super Mag. Neither of us mind the cost cutting measures (el cheapo wood, matte finish, etc) but reliability is luck of the draw on these guns. In the hundreds of rounds I've run thru mine, I've only had real difficulty extracting once. His, however, would have constant issues with crappy ammo. We'd have to bash the buttstock into the ground while holding the fore end to get the shell to come out of the chamber (careful doing this or you could rip the extractor claw right off the rim of the shell). All that being said, when he bought an 18" barrel to convert it to HD mode, it cycled like a champ. Go figure. After he polished the chamber of the other barrel it cycled well too.
Moral of this story: you might get lucky like me, or you might have to do some polishing to get yours to work right. I don't think regular 870 vs SuperMag has anything to do with it. The SuperMag felt just as smooth and swung the same as the regular 870, BTW.