Ensure that you are not using WAV. Plex cannot read metadata from WAV files. So all it can use are folder structure and file names, which might not be enough to identify the release – particularly if the folder structure is not following https://support.plex.tv/articles/200265296-adding-music-media-from-folders/
Look at One album being incorrectly identified as Various Artists instead of Alabama - #2 by OttoKerner
