I have the same problem with one of my students, a 14 yr old girl. She likes to come to the lesson and show me what she can do. she can play the Canon in D and a piece I used to play called "Swinging in Fairyland", but she is just at the beginner stage of reading! My big problem is that she doesn't seem to want to practice her lesson, only the things she already knows how to do. She'll come to the lesson and I'll say, "Did you practice good?", and she'll reply, "Yes", and proceed to play the Canon in D or the other one.
I would like to see some answers to this dilemma, as well! And, good luck to you!