Why won't iTunes download HD movies?
Open iTunes > go to Preferences > Downloads. Ensure that the checkbox which asks if you want HD footage to be preferred over SD is ticked. Also, check your storage and make sure you are using a screen resolution that isn't too far off from the default. If this does not work, turn off the computer, then press  shift + control + option, then press the power button (but don't hold). Then once you have done that, power on the computer. Some users who had this bug have reported that as a solution.