Hello itunesnotplaying,
Thanks for reaching out to the Apple Support Communities! From your message I understand that you are unable to play movies through iTunes on your PC. I know how important it is to be able to watch your movies.
You've been able to provide good information, which helps decide where to begin troubleshooting your issue. Follow these steps below:
1 - Quit iTunes on your PC along with any other apps running. Restart your Windows PC.
2 - Launch iTunes and try watching a movie: Play movies and other video in iTunes on PC. Be sure to test this out with more than one movie. (With the issue not occurring in music videos, you'll want to use movies during this troubleshooting)
3 - If there is no change, go to your purchase history to see if you can redownload the movie. If it's listed, delete the movie from your library, and download it again: Redownload apps, music, movies, TV shows, and books.
Problems downloading? If music, movies, or TV shows from the iTunes Store, Apple Music app, or Apple TV app stop downloading.
4 - If the movie will not play when downloaded, try streaming the movie also: If you can't stream or watch movies or TV shows from the Apple TV app, iTunes Store, or third-party apps.
5 - If you're unable to stream it all, check out this article for more troubleshooting of your iTunes for Windows: Gather and use iTunes Diagnostics in iTunes for Windows
All the best!