I'll take a guess. If this is a hammerless shotgun, and it was dry fired without the barrels attached, then the cocking ears that stick out the front of the receiver won't let you reassemble the barrels to gun. You need to push those two cockng ears down to cock the internal hammers. Without the leverage from the barrels, they will be hard to cock. Might need to push them down against the edge of a table (be careful not to mar table or whatever).