Have you used a pump prior to this? If you grip the foreend too tightly at the shot, the slide release won't get a chance to unlock. You can practice this by dry firing the gun, unloaded, of course. One or two dry firings won't hurt anything as you learn to use the mechanism. I always give the front wood a little shove forward, after the shot, to get the slide released. Different models of pump shotguns have different ways of releasing the slide after the shot-the model 17, and the model 12 Winchester seem a bit fussier than a Mossberg 500, in my experience.
If that isn't the problem, these guys are qualified to work on the gun:
http://www.diamondgunsmithing.com/I wouldn't be in a hurry to return it to the seller. Good copies of this model aren't running around in bunches. I wouldn't be a bit surprised if the gun, over 80 years old, simply needs a very good cleaning.
Best,
Ted
