"Get Album Artwork" doesn't work for some albums, yet they're in the store

I recently started using iTunes more often (after purchasing an iPhone) and so I've been going through my existing music and attaching artwork for all the albums I can. Sometimes there's issues with the specific way an album name is spelled/etc. that can cause "Get Album Artwork" to fail, so I've been looking up my albums on the iTunes store first, then ensuring the artist/album information matches. This works 99% of the time, but every now and then something won't work. Here's an example.

There's a series of compilations called "Hey Brother... Can You Spare Some Ska?". ( http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewAlbum?id=59528085) I own all four volumes of it, and iTunes has volumes 1 - 3 available in the store. However, no matter how consistent I make the file information between my existing mp3s and what's presented on the iTunes store page "Get Album Artwork" will fail. I'm not sure what's going on here, but whatever it is it's really annoying. Anybody know how to get this to work properly?