Make sure Plex has necessary file access (at least Read Only). See Plex FAQ #5.
See Adding Local Subtitles to Your Media.
- Make sure you have Local Media Assets enabled.
- Make sure the subtitle file is named exactly the same as the movie file, including any extraneous information such as [1080p.hevc.etc].
- Make sure the subtitle file has the correct language code. Otherwise it is displayed as “unknown.”
- After making any necessary corrections, refresh the metadata for the entire library.
If still having issues, please provide a directory listing, etc to show how files are named / organized.