Cover Art

I have hundreds of albums in itunes, with artwork. Some I have got through itunes others I have sourced and added. When I try to move to a flash drive for use in the car or into Windows media player, a lot of the artwork is not carried accross. I have tried to use MP3TAG to interogate the files and cannot see the artwork for these files. I have tried using itunes to embed the artwork using 'convert id tags' but this does not seem to make a difference. Any ideas please.

Isn't this a problem of iTunes? What about their forums?

You can see some threads here:
https://forums.mp3tag.de/lofiversion/index.php?t17200.html
or here
https://forums.mp3tag.de/lofiversion/index.php?t19719.html
or perhaps here
(external link)
http://dougscripts.com/itunes/scripts/ss.php?sp=embedart