If you are using the Plex Movie agent, try including the IMDB ID in the folder & file name.
Then Plex Dance the movie.
This works on my server:
/movies/Fantastic Beasts The Crimes of Grindelwald (2018) {imdb-tt4123430}/Fantastic Beasts The Crimes of Grindelwald (2018) {imdb-tt4123430}.mp4
If it still does not show up, then set the folder/file permissions in Synology File Station. Set them even if File Station shows they are OK.
- In File Station, right click on the folder containing the movie (or the entire library). Choose Properties.
- Make sure the owner is correct.
- Click the “Apply to this folder, subfolders, and files” box.
- On the Permissions tab, verify things are set correctly.
- Click the “Apply to this folder, subfolders, and files” box.
- Save.
- In Plex, rescan the library.