I'm no expert, so feel free to disregard, but my instinct is to agree that ADHD does not seem appropriate. If she was having problems paying attention, then she should be missing important facts and concepts in class, and those items should be showing up on tests. If she's all aces when she bothers to do the work, and she's not bothering to practice at home, then it stands to reason that she must be paying sufficient attention in class.
As someone who had a less-than-perfect record with homework himself, I can say that there's another, more pedestrian explanation: homework is boring. Furthermore, if she's still getting the grades she wants, and positive attention from the teachers, then her opinion is: "Why bother?" And there's some legitimacy to that position. If she's successfully avoiding homework, and successfully pulling down the grades she wants, where's the problem?
(Of course, we all know where's the problem, and it's coming not much further down the road)
It does sound like there's some executive function issue in there that's compounding the issue, apart from merely avoiding homework.
The first thing you probably need is for her to see this as a problem that needs correcting. A natural consequence would be to step back your scaffolding, and let her receive the lower grades she'd earned. If that doesn't get the reaction you're looking for, perhaps checking on her missing assignments once a week and having some sort of reward/consequence in place will work.
If it really matters to her, she should be able to come up with an executive system on her own to make sure she's keeping up with stuff. And if she struggles, that's when you can step in to help her design that system.
