You can read more about my problem here https://forum.synology.com/enu/viewtopic.php?f=193&t=139029&p=514616#p514616
So basically if I want to see new files I have added to my Synology DS218+ (drag and drop into shared folder) I have to create a new library folder to see it appear in Plex.
Scan library files and refresh all metadata does nothing.
I did a manual install of the Plex 1.10.0.4523 update.
So any idea why they aren't automatically showing up in Plex and why I have to create a new library just see a single newly added file?