Do what chippy says first. The inertia block in these guns is much lighter than say, Mirokus and the slightest amount of gungy oil or grease will stop it from moving freely. I would advise against using WD40 inside the Beretta action because it will eventually introduce the problem outlined below.
Alternatively, it could also be the cocking rod for the bottom barrel. If you take the barrels off and look at the front of the action there are 2 small rods low down on each side. You should be able to push them in a little way against a light spring which returns them to the stop position. Apart from the spring there should be no resistance to movement because if there is, the hammer has to push them forward which can prevent the striker from hitting the primer hard enough. Sticky cocking rods and associated cams etc are a gunsmith style strip and clean.