I still get this problem now and then, but it seems to happen much less frequently than it did after iTunes Match first rolled out. There's a few things you can try, and they have all worked for me on different occasions, so I can't tell you which one is guaranteed to work. You'll have to try them yourself and see.
1. If you're using iTunes Match on your iPhone, go to Settings > Music > toggle off both the Show All Music and iTunes Match buttons. Go back to the Music app and wait for the album list to clear and display the "No Content" message. You may try going to Settings > General > Usage > Music and deleting all songs locally, too.
Return to the home menu and double-click the Home button to summon the task manager, then kill the Music app. Now go back to Settings > Music and toggle the Show All Music and iTunes Match buttons back on. Return to the Music app and let the library repopulate from iCloud. You may try scrolling up and down the album view list for a while as the cover artwork images download to let them cache properly. If one is being stubborn and not appearing, press it to open up its track listing view and see if it loads in there. Hopefully it will show up.
2. From iTunes in OSX, right-click the album that's being troublesome and click Get Info. Even when the cover artwork shows up just fine from within my iTunes library, I've still had it not show up on iOS. Search Google Images for the artwork you're looking for and right-click and copy it to the clipboard. Paste it into the artwork field in the Get Info window and click Apply. Instead of simply right-clicking on the entire album if in album view, you may also try shift-selecting each individual song and pasting the artwork into them that way. I have seen it not work until I did it this way, so give it a shot. If using iTunes Match again, click Store > Update iTunes Match and wait for the changes to sync to your iPhone. Navigate to or play the album and see if it pulls the artwork down in the album listing view now. Otherwise sync the new tracks to your device manually.
Good luck! iTunes Match still has a few pesky bugs to iron out, but its definitely gotten better since the rollout in my opinion.
