Where can you get a barrel for a Baikal MP153?
Remington now imports the Baikal shotguns under their 'Spartan' brand. Barrels for the "Remington" Spartan SPR-453 are the same as your Baikal.
There are a lot of possibilities here. Does the other barrel fire if you set the trigger for the other barrel? If so, you may be holding the shotgun too loose, away from your shoulder, or have too much padding on. The Baikal in ejector mode has to, upon firing, trip the ejector sear. If you hold the firearm too loosely, the ejector sear will not trip and the next barrel will not fire.