D.,
What you describe is very typical of a child with ADHD. Many kids with this disorder have little idea how to plan over time (the hour or so you left her) have very poor ablity to self monitor what is successful and what is not, so they really don't know how to reproduce the good job they did last week, or how they ended up with an F yesterday either. They may have various processing defficits (sometimes processing speed, or verbal/math ablities) so they have difficulty with some areas that are extra hard for them to attack. They also loose track of time, and if her medication is not long lasting, she could be having episodes of lossing her train of thought, which is part of the disfunciton, when the child's idea or thought fails to continue along the millions of synapses in her brain (the tiny space between brain cells where chemicals carry our "thoughts".) Think how frustrating that is when it happens to you, and imagine what life would be like when it happend all day long!
If she is not receiveing help at school with this problem, she should, and you can request that the school step up and include a goal for her in an IEP or 504 plan, if she already has one of those. If she does not, this is the time to request both evaluation and intervention. Homework problems are a school problem, and could be the "educational need" that can get her services at school. This is NOT an issue related to her home situation or your parenting skills.
Your job is to provide her a place and time to do the work, and to give her the tools she needs to do it (medication being only one too for a kid with ADHD.) You can monitor her and remind her to keep working, and help her with questions when she hass them, but if all this fails, you should ask the school to step up. Beware: the school will be happy to let you shoulder the load if you are willing to do the job for them, so make it clear, in writing, that you expect for them to take responsiblity. She does not have to fail before she gets help.
If you have any more questions about how to get the school to help, please let me know. There also may be a parent mentor working at your school, and they may be able to help you navigate the system.
My best and first advice is this: Know more about your daughter than the school does and purchase your own comprehensive educational evaluation from a neuro psychologist. Second, if it did not happen in writing, it never happened.
Also, check out www.wrightslaw.com and read as much as you can about advocacy.
Good luck,
M.