So let me see if I've got this straight. This person is only in one game, but has more than one character in the game. Or do they only have the one character in that game. At any rate, you need to talk to them and ask them why they keep promising to scene and then never show up or reply. Most of the time if a person keeps making promisises like that or is really slow to reply to a thread/scene, it's a sign that they're not interested in the storyline. I'm not saying that's the case here, but that's been my experience.
It is frustrating, but you need to talk to your storyline partner and try to get to the bottom of things. Good luck.