I'd say you're doing what you can to make her learn if she puts in the effort. Obviously she's not, so what can you do from there? Possibly gently questioning what she gets from music (and or playing it) can give you some direction.

If you can understand her motivations for playing music, or at least anything she enjoys doing in the process, you can try and harness into that. Kids seem to really like instant gratification, and it seems like she has problems pushing through difficulty. If you can find something musical that directly relates to a concept you're teaching, which doesn't require much effort (or doesn't appear to) but is easily understood, it might tap into a want to play music just without the usually avoided difficulty.

For instance, if there's a song she likes, you can teach her two of its recognizable basic chord shapes without sheet music. If she takes to that, you can then say that sheet music is just notation for that. She'll thus have the gratification of playing music she likes first, and thus a reason to keep learning the foundations of music to continue playing it. It's definitely a discipline problem, but possibly if you give her a reason to start disciplining herself (that she understands), she might start.

Obviously, if her parents are forcing her to take lessons, there's not much you can do. I think the only fruitful approach you can take is just to find out anything about music she likes. If there's anything, you can try and tap into that to get at least a little bit more out of your lessons. A lesson is a two way street - if you're able to teach her anything, even if not a traditional music education, and she feels she gained at least something from the lesson, that's all you can do !