I'd say a bit stiff is normal. Unless you're talking about an 870 Express and then I would think it would be even stiffer.
If you hold the release in on yours, with the muzzle up, and shake it down once, will it open? Will it open if you bang the pad on the floor? Don't worry, it'll get better.
My Exp. Mag. took a thousand or two cycles to smooth up, but it still wasn't a Winchester Model 12. I did help it along some with some Flitz polish used sparingly on a few of the moving parts. It would have smoothed out eventually without the polish, but I needed it for duck season and didn't have time to break it in by shooting it every other day for a couple of weeks.