This usually comes down to file permissions or naming & organization.
Make sure you follow the Plex naming & organization guidelines. Plex Dance the movie after making naming/organization changes.
Also, some mp4 files have bad embedded metadata that can hinder Plex’s ability to correctly recognize and display the movies (or TV shows).
To configure Plex to ignore embedded metadata, edit the library, choose Advanced, and deselect the Prefer local metadata option.
FYI, Subler is a very nice Mac based tool for remuxing media. It can also import metadata from several online sources.
Plex Documentation → Your Media
→ Naming & Organizing Your TV Show Files
→ Naming & Organizing Your Movie Files
https://support.plex.tv/articles/201543057-why-is-some-of-my-content-not-found/
