Linux has a limit on the number of directories it can monitor. When exceeded, Plex does not pick up new media added to the system. I do not know if this applies to unraid (I run Plex on a Synology NAS).
If this is happening, you will see iNotify errors in the Plex log files. Look in Plex Media Server.log (wraps to .1.log…5.log). Aug 06, 2019 17:15:24.569 [0x7fcb07fff700] WARN - [Notify] Received unexpected inotify event: 8192