CHANGER: The Crystal Doesn't Work on Bobby?

Rachel leaned towards Bobby, clutching her magic crystal and smiling at him. But then her smile turned into a frown, and then a look of confusion. Bobby wasn't sure, but he thought that maybe her power wasn't working on him for some reason. But why? Had her power run out? Or did it have something to do with the fact that Bobby was a teenager and not an adult? Or some other reason?