Just been playing around with iTunes and iMatch - most of my artwork has ended up on my devices.
For those that haven't ended up on there, I've just re-uploaded a smallr version of the artwork.
Seems to be caused (as others have said) by large image files. To combat, I did the following, which resulted in the artwork appearing on devices...
1) remove artwork from offending track(s)
2) update iTunes Match in iTunes (Store > Update iTunes Match)
3) add new lower-res artwork to offending track(s)
4) update iTunes Match in iTunes for a second time (Store > Update iTunes Match)
5) close down iTunes on all devices
6) re-open iTunes on devices and browse to the song(s)/album(s) - you'll see the whirling wheel at the top of the screen, showing something's being downloaded.
7) play the song.
8) stop the song.
9) navigate back to the album/song list and the artwork should appear.
10) go back into the song(s)/album(s) and the artwork should finally be there.
Hopefully this will help someone who hasn't already found a solution - apologies if the above has been posted in detail. This worked for me, so thought I'd share my solution.
Looking at it, though, this is pretty poor for Apple. Surely their system can handle higher-res artwork without spitting its dummy out...
KG