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For more information on this, take a look at:
iTunes Store: Troubleshooting iTunes Match
If some songs or albums downloaded to an iOS device from your iCloud library are missing album artwork, try the following:
- Make sure you have the latest version of iTunes. You can download the latest version for free.
- Launch iTunes on your computer.
- From the iTunes menu, choose Advanced > Get Album Artwork.
- Once iTunes has completed processing the album artwork, you'll need to update iTunes Match.
- From the iTunes menu, choose Store > Update iTunes Match.
- Once iTunes Match has finished updating, launch the Music app on your iOS device.
- Locate the songs or albums that are missing artwork, and delete them from your device by swiping left or right over a song. To delete an album, swipe left or right over the album on iPhone or iPod touch. To delete an album from iPad, tap and hold on the album cover until you see the delete button or "X" in the upper-left hand corner of the album cover. Tap the "X" to delete the album.
- After you've deleted the song or album, download the item again from your iCloud library by tapping on the download icon.
Best of luck,