What might be causing Plex to mark selected track as Unavailable?

Two days ago I was watching a TV series on Plex (via NVidia Shield), yesterday it started to say it was Unavailable, this includes episodes I have already watched.
The drive is mounted OK, because I can watch other series from the same drive.
The files are OK because I can play them through other software.
The formats are OK because I watched some episodes two days ago, episodes that are now being marked as Unavailable.

Its not just one series affected. It appears about half the content of this mount is now unavailable in Plex but ok through other software.
Rebooting and refreshing content has not fixed the issue.

Any ideas what would cause this to happen to selected items like this, and how I might fix it?

This is affecting tracks across multiple mounts, which connect to multiple physical devices. Its like it has randomly selected halve my library not to play:-(

Problem solved. It was very odd. I noticed the two affected drives where the to network attached ones. So I unmounted and remounted. Now it works. I wonder why it was only selected tracks it was affecting on these drives. Maybe it doesnt like my choose of TV!

I am also having this problem and the solution works, but it keeps hapening every week! I can’t keep mounting and unmounting the drives just to keep Plex happy. This is a Plex specific problem. I also use kodi the same way and it continues to play the same files from the same drive even though plex says it unavailable? Does anybody have a longer lasting solution?