I have a great many Albums which have the wrong album art. When I tried to use ‘Match’ it seems it only uses a search tool that I have not seen before. So despite having the exact meta data for the album it finds the wrong image. It is really tedious doing all this manually as you may imagine.
Is it possible to select another tool?
Find the cause of the illness and cure it, instead of painting over the rash. 
Are your albums organised in folders? (one folder for each album)
Do these folders contain album art in separate files? (sometimes this album art is ‘hidden’)
Do the higher-level folders contain album art files as well? see https://forums.plex.tv/discussion/comment/1173370/#Comment_1173370
Inspect your song files. Do they have cover art embedded?
Do they have the right cover art embedded?
Check with a meta tagger software e.g. mp3tag
