Album Art not being created

Just installed Plex on a brand new nvidia shield. All of my media is stored on a synology NAS. I’ve managed to connect the shield to the NAS and add the relevant folders on the NAS to the plex libraries.

Via a plex client, I can play movies and songs and view photos. So far, so good.

However, there are a few albums that don’t cover a folder.jpg image, and so plex just shows the generic folder icon. I’ve tried to refresh the album art and I’ve also tried to Fix Match. When I use Fix Match, I can see the correct album image in the popup, and after selecting it I see a message about “Downloading metadata…” at the bottom of Plex. However, nothing actually changes, and there is still no album art. I’ve also manually checked the relevant folder on my NAS, expecting/hoping to see the folder.jpg, but there is nothing there except the actual tracks.

Am I missing something obvious here?

Thanks for the help!