Never mind, I figured it out. In fact lack of ammo was the first thing that came into my mind into the first place, but since one of my characters had started with a bow but without arrows, and still there was a full quiver depicted along with the character in the character description screen, I thought that maybe missiles were supposed to exist along with the weapon. Then I found some at a merchant's and I bought them and everything went fine. But thanks for answering.
This game is too cool for school, I've killed a dragon.
