Playing more -- or less -- with her could make the difference. Asking better questions about her matches can help, especially if you reflect a focus on her enjoyment and personal progress more than on her scores.
She might also be missing her friends. If none of her friends play, try to find a group lesson with some lively, fun kids her age. There's a good chance she'll take a quick liking to them and be eager to see them at the next lesson.

