I am using a Galaxy Tab with an additional SD card, and playing music on Google Play.
I can see the album covers in the Art Grabber App and it looks like I have downloaded them. However, not all of the album art shows up in the music playing app. I have tried "rescanning" the SD card, but that doesn't work. Does the program know I have two SD cards? On the tablet, an attached SD card shows up as EXTSD card (as opposed to the Internal SD card that comes standard with the Tablet).
The only reason I mention this is because I saved all of my music to the external card, and it looks like the album thumbnails are saved in a folder on the internal memory. But that wouldn't explain why some of the albums covers show up and some don't.