Plex not finding some seasons of shows after struggling with it for a while

[pshanew]

Does this similar issue apply to your situation?

Synology: Cannot see any folders in share when adding media library

There is a long standing problem when using the pre-created Video share. The conflict is that DSM (Synology) loves to override any and all permissions at a whim. It’s done in such a way that even when PMS was running as the administrator, it would still lose access. The solution to this is: Create one or more free-standing shares of your own. (Movies, TV, Music, etc) As you create these share(s), give user “Plex” permission to R/O or R/W the share Now move your media there. You will never…

At any rate, given that your naming appears to be fine, it is most likely a permissions issue. Ensure that the Plex user has (at least) read access to the shares where your media is stored.