I have an old Remington Model 11 (same action). Forgive my lack of proper technical wording, but there's a 'thingy' that catches the bolt in the rear position after the last round is fired. First thing I'd do is pull the bolt back slowly by hand while looking to identify the 'thingy' to observe if it moves into position or has been damaged or sheared off by somebody shooting magnum rounds in the A5. The part is in the inside of the action, near the mag tube, and is connected to the button that you push to release the bolt after reloading the gun and to get a round into the chamber. I had that same ejection problem while goose hunting in 2 degree weather on the Maryland Peninsula many years ago. I think the old shotgun was shocked to be out of Texas and in cold weather.
That may or may not be the problem, but it's the first thing I'd look at if it was my old shotgun having issues.