Problem with Spotify Artwork

I've got a strange issue and I'll see if I can explain it here.

I'm using the Spotify Protocol Handler plugin and the official Spotify app on Logitech Media Server Version: 7.9.0 - 1483028105.

When I browse Spotify (using Squeezer app or web interface) I can see all the spotify artwork. However, once I add this to my own playlist the artwork for many tracks disappears.

I have a couple of custom apps that need to access the artwork so it's a shame that this is getting lost.

Does anyone have any ideas what's happening?

I appreciate I've given little information here so I'm happy to provide any other info that anyone thinks may be relevant.