Hi, I think with the Unitiserve the artwork would have to be referenced within the Unitiserve properietry metadata files that it generates for Wav ( amginfo.xml). Of course the Naim Import Metadata tool within SongKong works the other way letting the metadata within those proprietry files go into your files.
But it may be that Unitiserve recognizes the files if they are stored as a folder.jpg within the album folder so you could check that and when using SongKong ensure that Save front cover art to filesystem is allowing the artwork to be saved in folder ( as well as the default of writing to the music files)

I would start by comparing two folders with artwork, one that displays and one that doesnt and determine the difference,