Artwork missing (randomly) - why?

Are those albums from your personal music library? If so, the Sonos app might just be taking a while to load the album artwork files because of the size of the files and/or the size of your music library. Or there might be no album artwork associated with those music files.

Library artwork is fetched via the controller’s Associated Product. With a less-than-ideal network path -- extended/weak wireless hops -- this could run into latency issues.

The controller has very limited patience for library artwork. It will display -- and cache -- a blank. Force-quit of the controller and restart will often recover the missing art.

If the local library is outside the local subnet, addressed by IP, this can sometimes cause other issues.

Yes.  It's not a huge library.  Sometimes there are only a few greyed images, then on another occasion there are many.  It's not consistent. 

Where is the library stored?

Next time there’s a problem note the Associated Product IP and figure out which player is being used. You might see a pattern.

There's only one hop - music on router USB drive. 

What's an associated product? 

Look in Settings/System/About. The Associated Product is the node the controller uses to obtain information from the system, such as the library index. It also proxies requests and responses to obtain library album artwork.

If the Associated Product is a wireless node the round trip to fetch art from the router’s USB drive could require at least four hops over a not very good connection. 

If I understand you correctly then the ap is a connect.  Traceroute reports one hop. 

Traceroute? I’m talking of physical hops across SonosNet. 

Eh?  What does physical hops mean? 

castalla,

Controller → associated player → USB drive → associated player → controller.