Can no longer add artwork to albums in iTunes

I've had no problem in the past adding artwork to albums I've added to iTunes from CDs. Starting sometime after July 2018 I can not longer do so. Everything looks fine as I Get Info, go to Artwork, click Add Artwork, and choose a JPG file. The art shows up fine in the Artwork pane but when I click OK it doesn't appear in the album view. If I go back to the Artwork pane it's gone. I'm running Mojave 10.14 and iTunes 12.9.0.164. Again, I've done this successfully for years with hundreds of albums. Any help would be appreciated.