iTunes suddenly will not let me add manually album art I am running High Sierra, updated, with iTunes 12.7.2.  My Music and Album Art is on an external HD.  I rip my own music into AIFF files, then manually add the album art.  This afternoon all of a sudden it will allow me to go through the motions, but the album art does not appear in iTunes.  I have checked the permissions for some of the files and they say I have read write permissions.  Please do not suggest downgrading my music.
Sure, it's a year late, but I had this problem which lead me here, so someone else might, too. I had been adding cds to iTunes and manually adding covers, then one day it would not save the cover image. Here's what I did: 

I turned off the Airplay on my iTunes.

This worked. I could save cover art again. Also changes I made to album info (fixing the album title, etc.) reverted back. Apparently, even though it showed these edits as changed, they were not saved when my speakers were set for airplay.