It would be my guess that it sounds like when you pull the pump back the loaded rd gets kicked out the bottom, so it'd be my guess again that something is wrong with the cartridge stop.
Take the gun , barrel pointed away from you , flip it upside down , push down on the elevator "floor plate" and inside the gun near where the mag tube begins should be a bar like thing on the side of the receiver, possibly on the same side of the gun opposite the ejection port , but maybe not "unsure of the mossberg" anyhow is this bar "cartridge stop" stuck in? Under normal circumstance with the gun working properly you should be able to push this bar inwards and have the shell in the mag tube pop out , so perhaps that is sticking for what ever reason , or worn , or etc,etc
