Hi all,
I had the same symptoms with iTunes 12. Artwork wouldn't even sync to my iPhone anymore which was the most annoying part. Here is how I got everything back again. NB: My music library is stored on a NAS as well and if iTunes couldn't find the artwork I had the artwork .jpg previously embedded via "Get Info"/Cmd+i and paste into the artwork field.
Step 1: Getting Artwork to show in iTunes again
What finally (after many Cmd+i/Enter sessions and deleting the artwork cache) did the trick for me was the script "Re-embed Artwork v2.0" from Doug's Applescripts for iTunes: http://dougscripts.com/itunes/scripts/ss.php?sp=reembedartwork. This brought all the artworks back and iTunes hasn't forgotten them so far. NB: You can select multiple album at once and apply the script however the script will abort if there are missing files marked with "!" and you have to start again.
Step 2: Getting iTunes to sync to iPhone
(Before I tried unchecking "Sync Music" and sync and checking back on again but it didnt work)
Syncing with the iPhone after Step 1 solved most of the issues and artwork was updated on the iPhone.
The ones that still didn't work I cleared all artwork (cmd+i > Artwork tab > select artwork > cmd+del (if you are sure iTunes can "get album artwork") OR cmd+c to copy into clipboard) and then cmd+del > OK. iTunes should save the changes in each song of the album.
Next, sync with iPhone. Each song will be writing without the artwork to the iPhone.
After that, copy the artwork back into the album. Select album > cmd+i > artwork tab > cmd+v > OK.
Finally, sync music again.
NB: I could see that each time iTunes rewrote the whole file onto the NAS with ~7MBs over wifi. The same read speed when syncing to the iPhone.
What might be the problem why 're-embedding' or cmd+i& enter don't work is that the file size is not changing. Don't know how iTunes decides that a new version of the files is present and to trigger an actual copy. Even deleting iTunes own artwork, saving the file (without sync step here) and re-embedding it seems to result in the same file again which iTunes won't sync back to the phone. Keeps me thinking that writing an uncommonly used ID Tag into all files (might take a while) could trigger to copy all files with the next sync. After that you could delete the ID tag again. This could save the time to go through individual albums and sync without and with artwork again.
Hope that helps to solve this nuisance for most of you 🙂 Good luck!