Check some other usual suspects.

First, try to chamber fired shells from problem gun in your gun. Possibly the problem gun has a chamber irregularity, and the expanded steel hulls are sticking. Try also firing the gun with brass headed ammo, like STS or AA. Rio IS good ammo, but the 'brass' (which is steel) is higher than STS or AA shells and if it's expanding into a rough chamber it will be hard to extract.

"The price of good shotgunnery is constant practice" - Fred Kimble