I have no clue how or why your doctor believes that until he tries it. Dr. Charles Parker would say (link later on) that she is at the bottom of the window and you simply increase the dose. This is true for any medication that has been working. He also says that Vyvanse is a bit difficult to pinpoint when it quits working because it is so smooth. It doesn't have the sudden zap or let down like the instant release meds. And, of course, you could simply do an instant release adderall in the afternoon. But, I would definitely first try uping the dosage (and maybe looking for a new doctor).
Anyway, here is a link to Dr. Parker talking about the problem of "ADHD medication dosing, not enough". Its also worth taking the time to view his other videos on the subject. I have bought his book and its great. Maybe you should buy it and give it to your doc. The link is
https://www.youtube.com/watch?v=Z8UVM1tPXHU
Hope this helps. Please post if you have any questions.