Once completed I closed and re-opened iTunes to find that a lot, and I mean a lot of cover art was missing. I wasn't so much worried about the more common cover art that's available by using iTunes' built in-downloader but by the cover art that I had personally added over the years to the more obscure albums in my collection. I was not prepared to spend hours hunting this artwork down again. The good news is that eventually I was able to get all of my cover art back. Not any one task fixed it, but after a lot of trial and error, it was all there. Here's what I did:
1. Turned off iTunes Match - iTunes > Store > Turn Off iTunes Match.
2. Used the iTunes cover art retrieval option - iTunes "Advanced" (top toolbar) > "Get Album Artwork". (I received a notice that 21 covers could not be found).
3. Closed and reopened iTunes (several times)
4. Clicked on cover art that was missing (and it would appear)
5. Cmd+I/Control-click (right-click) on missing cover art to open the "Get Info" screen (and it would appear).
6. Turn iTunes Match back on and click on "Update iTunes Match" in the same drop-down menu
After doing those various things all of the cover art was back including the cover art that I had added myself.