What playback devices are you using with the Boost Pass ?
When you go into the NowTV App on your devices under the My Account section it should show the HD Boost Pass.
You shouldn't have to log out of any devices that are HD Boost compatible where next to the movie information there should be a HD symbol or if you press the pause or info button the pop information screen should show HD 5.1 SS if the film is available in HD and surround sound.
Should none of your NowTV Boost supported devices are showing the above then see if signing out of the the NowTV App and logging back in triggers the Boost Pass.
Have a read of this help article for further information and what devices are Boost supported.