If running PMS on Linux, you could be exceeding the number of directories Linux can monitor.
If this is the case, in Plex Media Server.log (and .1.log to .5.log) there will be ERROR messages mentioning “no space left on device.”
Here’s the fix for Linux & Synology. Will be similar for other Linux based systems (QNAP, etc).
Linux fix: Increase the number of directories Linux can monitor (inotify) for PMS
Synology Fix: FAQ 16: Increase the number of folders Plex can monitor
