Chiming in: (using latest versions as of august 20 2013)
Slightly different but similar problem. I spent inordinate time tagging and adding art. Yet some albums show no art in google music. There doesn't seem to be rhyme or reason. The artwork shows up fine in when i look at the files in windows file browsing and in media monkey. Every forum about album art problems has the same "solution" - edit the tag in google play. This is worthless advice. When tags show up properly in other apps, what is the point in having to re-tag things.
Yet every few dozen albums are blank and when i go and edit tags on my computer, making SURE i get everything right (inluding embedding the art in the tag), I then delete the album in google and upload it again from scratch. But the only thing that works is going through the google album editor and manually selecting the artwork.
Tagging has been a headache in all apps since day one. Too many standards, not enough consistency, not powerful enough group editing. I'm getting really sick of every single new music app I use having a NEW set of issues with tagging.
Theories: (untested and wonder if anyone knows)
1) file format for the artwork? I use jpeg whenever i can though a few png files may have snuck in. Although when i re-upload problem albums I make sure to use jpg. Is google super sensitive to file format? When i manually select the artwork via google music, i am choosing the same artwork that i originally tag with but google may be converting the file as it uploads, thus explaining why it works that way and not when you edit on the home computer. Various fine-grain differences in jpeg formatting is my best guess for this intensely annoying issue.
2) art file size too big? For some albums I go out of my way to choose larger high res art to tag with, perhaps google doesn't like larger filesizes? Again, when i manually pick the SAME large artwork via google, it works, but again google might be converting the image as it uploads.
3) fine grain formatting differences in mp3 or other files? Even though my 'problem' albums are in mp3 format, tiny encoding differences may cause problems.
Also, I just wish when something goes wrong that a simple error message would pop up "Hello, this is google, your album art did not load because blah blah" would be nice. then i could get somewhere solving this.