I had this issue, i imported music from itunes library & others from my hard drive. Some artwork displayed on my Admundson & others didn’t, it was driving my crazy. All artwork i added was
within the max resolution & file size. I discovered Mp3tag software, it was very easy to use & free, i then discovered that there were 2 pieces of album artwork on the albums not displaying, one left over from iTunes & the other correctly sized one i had added.
I clicked on the offending artwork & deleted both then copy & pasted the smaller resolution artwork back onto the album.
Now all artwork is displayed & correctly tagged to each track rather than the album file only.
you need to save after adding artwork & software will update every track in the album.