Just an idea based on being around 21's some.
Is the gun hard to open in comparison to other guns you own? The reason why it's easier to open after only shooting one barrel might be due to only having to cock & reset things on one barrel as opposed to doing it on both barrels. The gun might need a good cleaning of the action. Does it open easier on either barrel after firing a single shot, or have you tried shooting the other barrel first and see how it opens in comparison to the other barrel.
On the outside chance there might be a mechanical issue like a broken hammer spring on one barrel that can still fire that barrel but might be jamming in the bore of the action for that spring. There's lots of variables but without the gun in hand it's hard to say one way or another what's actually going on if anything.
