A lot of my albums in Loop do not show album artwork when viewing my library on my iPhone and in the app for the Mac. This was a problem a while back and it seemed to be fixed for a while and now the bug is back. Most albums have artwork but some do not. My original files in iTunes ALL have album artwork.
Also, some albums that do have album artwork showing up in Vox on the iPhone do not show the artwork in the lock screen when a song is played on that album. Normally, if the album has artwork and the album is playing, when you view your lock screen it shows the album artwork and info and you can pause the song. But a lot of the time, no artwork shows up and if I look for that album in Vox, it does show artwork.
I am attaching 4 images. One is part of the library with several albums that should have artwork, not showing any artwork. Another is the lock screen playing a song from an album and not showing artwork, and the other is that same album with artwork in Vox. The last is another song playing and the lock screen how it should look with artwork.