Ultraviolet is still relatively new, and not all UV movies are supported in any given app. For movies that say they are unsupported in the nook video library app, that generally means they are not available in nook HD format. You can download either the Flixster app or the VUDU app in the nook app store, and watch them via those apps in SD format.
The VUDU app also has a greater variety or sorting mechanisms for viewing your videos. If you download the VUDU for PC and Flixster for PC application onto your PC, you can also convert most of your non UV, non-Blu-Ray DVD's to ultraviolet format for $2 (SD format) or $5 HD format; all Blu-rays cost $2 to convert.
you can try loading the flixster app onto your nook from the google play store, and see if it will allow you to download or stream all your movies
I've installed MX Player and it plays all the movies I've tried. Sometimes, though, you need to change it from D/W to S/W decoding. Hope this helps.