Remove the cover art from the audio files and try again.
Save the cover as cover.jpg into the same folder.
Don’t forget the ALBUMARTIST meta tag.
Use a folder structure like in here: https://support.plex.tv/articles/200265296-adding-music-media-from-folders/
And make sure that every track in every album which is stored inside the same Artist folder is tagged with the exact same content in the ALBUMARTIST tag.
(If there are single tracks with guest performers, use the regular ARTIST tag to note their presence.)
