Revisiting the Artist and All Artists Index

[keithsonic]

I have just imported over 3k artist images into indexArtwork from LastFM via JRiver. I had a success rate of about 95% with most mismatches due to differences in how artist names are presented. However, I have two intractable problems I can’t resolve.
I have two discrete artists Chelsea Wolf and Chelsea Wolfe, both in separate folders and correctly tagged in album artist. However, MS shows the latter image for both artists.
More intriguingly I have separate folders for Christine McVie and as her maiden name Perfect yet MS shows the McVie image in both cases.
All metadata is tagged in JRiver and this shows the correct images in both cases. All images are jpgs so I am stumped.
Any suggestions.