I'm not a coach, so add some salt. From what you've said, she seems talented. If she's not being challenged, I could understand how she could lose motivation. You could ask her if she'd like to learn some skills that 13-15 yo players are learning. If she's struggling, try something else. Feeling challenged is key in staying motivated in anything we humans do. Give her more opportunities to take on challenges to her game.
As always, my point is moot.