Greyed out album covers

Why are some of the album covers greyed out in my library, but when I click on them they appear and play as usual.  At first I thought they were no longer available, but in every case it was just the album cover image that didn’t appear properly (desktop and desktop app versions).  In the ios app, the main cover image remains greyed out when clicked on (but the cover as it appears next to each song is visible).