Two things coming to mind:
- could be a permission thing (not sure however if/how this is handled on the Asustor NAS)
- there’s certain circumstances where Plex will ignore files – it’ll e.g. ignore files that have a tv-show naming pattern in a movie-type library. Plex will also ignore files in unsupported formats (such as disc images / ISOs…)
https://support.plex.tv/articles/201543057-why-is-some-of-my-content-not-found/
