Although it’s happening a bit less recently, every once in a while when I add new media to Plex, the metadata does not appear. When this happens I first try doing the usual things like manually refreshing the metadata and emptying the library trash. If it’s still not working, I’ve found that manually deleting the cache files from the computer running the Plex server seems to almost always fix the problem.
To manually clear the cache, navigate to the cache directory and delete everything inside. For Windows, the cache directory is located here:
C:\Users\username\AppData\Local\Plex Media Server\Plug-in Support\Caches
In macOS, the cache directory is here:
~/Library/Application Support/Plex Media Server/Plug-in Support/Caches
(I do this often enough that I have also created a shortcut to the cache directory on my desktop.)
After deleting all the cache files, refresh the metadata for your media, and it should hopefully work.