If an opponent is at the limit of their range (they will not follow you more than a set distance from their starting point), they will not take damage.
A couple people have also reported specific opponents being invulnerable, but IIRC leaving the area and returning would generally fix the problem. A couple these cases were with goblins the start of the fjords (a goblin chief near the teleporter IIRC); one person left and returned later, while another just skipped that section of the path by jumping off the mountain, and when they worked their way back up later, didn't have any problem doing damage.
