I have spent hours trying to solve the few random missing album covers from my collection and I am starting to get closer to defining the issue.
Initially, I assumed that this was an issue with the artwork or the tags so I have triple checked everything, even re-exporting JPEGs in Photoshop in case there was a format issue. Nothing helped.
Then I ran a test, writing down the name of every album missing its cover. Around 10 in total (out of 1,200). Then I deleted the Audirvana database and re-imported the music library. This time, a different set of albums was missing their artwork. Some with earlier problems were still not showing artwork, but more interesting was the fact that some which were working before stopped displaying artwork, and some which were not displaying artwork before suddenly worked again! This seems to demonstrate that there is not an issue with the files in the library, but an Audirvana database issue of some kind.
Furthermore, all of the albums which sometimes don’t display covers are multi-disk albums, although most multi-disk albums are working fine so being multi-disk alone is not enough to cause the issue.