When we ran into this, it was because the incorrect video driver was being installed. Now, we were injecting video drivers ourselves via DISM, so it was easy to fix - pull out the bad video drivers, ensure only the right ones are left - voila - we're getting max resolution. And remember, we did NOT use DA, so if you're using DA... you could check the DA logs to see what driver is being used? Maybe you need to pull out an older one and add in a newer one?