Mine was a problem with TV show metadata not loading.
The following fixed the issue for me:
On Mac OS X plex stores several files in the User Library
First Quit Plex Media Server (make sure no one in the house may be watching something ;) )
either use Go To Folder and put in ~/Library/Application Support/Plex Media Server/Plug-ins/ ,
or in the go menu, hold down Option, and select Library then browse to the folder
In there delete "Framework.bundle" and "System.bundle"
If the problem only seems to be affecting TV or Movies, then you could also delete the appropriate plug-in (in my case TheTVDB.bundle)
When you start-up Plex Media Server again, it will replace the ones you just deleted.
In my case, this immediately solved the problem with Meta Data not appearing (or downloading, not sure which was the issue)