Try to explain luck in terms of it's mathematical nature, in a way that is pretty easy to understand. It sounds like she may "believe in luck" in an irrational way that lots of people do...like some mystical or intangible he's-got-it-and-I-don't and vice versa way.

That might help a little bit. Also, try to remember that the way she plays is supposed to be fun. Getting upset is not.