An easy fix for some iTunes Match glitches
I found that some cover art would not display on my iPhone, even though the same album would show cover art on my iMac and on the MacMini, from which it was uploaded to iCloud. Other problems occurred, too weird to describe here.
Investigating the problem, I found that this was caused by some "stuck" sync attempts, trying to download songs from iCloud to the iPhone.
Fix:
1. disable iTunes Match on the iPhone.
2. Delete all music from the iPhone (right swipe, click delete)
3. some music will remain and can neither be downloaded nor deleted. These are the "stuck" items that cause the problems.
4. Attach iPhone to iTunes
5. Select your iPhone in iTunes, then click on "Music".
6. You will see the "stuck" songs, greyed out. Most of my problematic songs showed up twice or three times, with a non-moving sync indicator in the first column.
7. delete these songs (select all, right click, delete). I had to do this twice for some unknown reason. Force a sync and check again, to verify that they are indeed gone.
Once the songs are gone, disconnect the iPhone from your computer. Check the music application. It should display no songs. Now, go to settings and re-enable iTunes Match on the iPhone. Wait until the busy indicator disappears. This may take several minutes, depending on the size of your music library. I assume that this is the time it takes to download cover art and metadata, needed to display the list of albums.
What is displayed on your iPhone should now match what is in the cloud.
At this point, I am just guessing, but it seems that the iTunes Match code on the iPhone is not yet 100% bullet proof, and that it can sometimes become confused by a combination of intermittent network connectivity on the iPhone and updates to the iCloud library. Once a sync attempt gets stuck, the affected file seems to be locked and will never display the proper cover art, sometimes affecting the display of the whole album.
I will keep an eye on this. Probably the code will become more robust over time.