There is not.
There are many reasons why a video file on your media storage might not be (visibly) represented in Plex.
Start by enabling the “Duplicates” filter in your library. (Switch the library into “Episodes” view mode when it’s a tv show/series-type of library)
Then look at the Plex media info of all those items which have a duplicate indicated. This will help you find naming and folder organization issues. Here’s documented how it’s supposed to be: https://support.plex.tv/articles/categories/your-media/
Then continue here: https://support.plex.tv/articles/201543057-why-is-some-of-my-content-not-found/
Additionally, you might want to check out/bookmark: Synology FAQ - Questions, Answers, and How-To's
