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If media is not seen, check permissions. User Plex must have permissions to read it.
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Incorrect matching is 99.9% a naming problem:
https://support.plex.tv/articles/naming-and-organizing-your-movie-media-files/
https://support.plex.tv/articles/naming-and-organizing-your-tv-show-files/ -
On Synology, using the default Video share is problematic. Best advice is to make discrete shares which you control and plex has been given permission to read.
Here is how to verify user Plex has permission to read the media.