https://support.plex.tv/articles/naming-and-organizing-your-movie-media-files/
As @Andreas_B mentions, make sure you follow Plex’s naming & organization rules. Use the movie name as displayed at TMDB or IMDB.
Plex generally recommends placing each movie in a separate folder (required if you want to add extras, cover art, etc).
Consider adding the TMDB/IMDB ID to the file names. That tells Plex exactly which movie to match.
Plex Dance the movies after renaming / reorganizing. Move them all out of the view of the Plex scanners. When returning, move them back one at a time. It will make troubleshooting easier if one of them is not properly recognized.
Example:
/Movies <-- Folder added to movie library
.../The Fast and the Furious (2001) {imdb-tt0232500} <-- MovieName (year) {imdb/tmdb id}
....../The Fast and the Furious (2001) {imdb-tt0232500}.ext <-- ext=mkv/mp4/etc
.../2 Fast 2 Furious (2003) {imdb-tt0322259}
....../2 Fast 2 Furious (2003) {imdb-tt0322259}.ext
