Since she was getting those high scores on the practice tests, this doesn’t seem to be a content issue. When my son took the Math II subject test, he was very surprised with how different the test conditions felt than when he was preparing. This is always the case with standardized tests, of course, but he said with the subject test it felt even more different for some reason. During practice tests he was finishing with 15 minutes to spare and in the real test he barely finished. So I would say see if there are ways she can more closely mimic real test conditions when she is practicing. Maybe have her go to another location, maybe have a friend of yours fake proctor it? Maybe even remove 10 minutes from her time alloted during practice tests?