iTunes Artwork Exists, But Won't Show Up

Recently, around half of the artwork for my albums disappeared and has been replaced with the default grey artwork when I choose album view from the view options. The artwork hasn't been removed completely, as when I play a song in the albums with the error, the artwork is displayed in the slim bar at the top of iTunes. The artwork is also displayed when I go to the 'get info' screen and select the artwork tab.

I have tried multiple things to resolve this issue. I've reset the album artwork cache, re-installed iTunes, restarted my computer, even updated my computer. The problem still persists, no matter what I do. I even tried embedding the artwork in the audio files themselves, which didn't work either. I've become very frustrated as I've spent countless hours compiling artwork for all of my albums that I've imported from my extensive CD collection, and I haven't found any workaround to this issue on the forums - even though multiple other people have had the same problem. I've included screenshots of an album that has this issue.