Do you know the person you bought it from? Could you inquire with them, what type of ammo they've used and if they encountered firing issues?
I could be wrong, but I suspect the action is not cycling all the way.
Is the Fiocchi buckshot full power or low recoil? Next time you take the shotgun to the range, try bringing different, full power ammo. Load a single round and see if the bolt will lock back after firing.
I can somewhat replicate the issue you're having with my Taran Tactical M4, but that's if I'm firing really fast and if the shotgun is dirty. I can pull the trigger before the bolt goes into battery, defeating the disconnector.