Anything weird happen entering the boss room (sounds not triggering, animations off etc)? The first time defeating the boss were you also dead? What happened when you spoke with the NPC? Did you hear a quest sound effect trigger? Was it correct dialog? Did he successfully get out of his cage? Anything weird happened before you entered the monolith that you can remember that could have caused the quest to break?
As much information here you can give me would be great.