iTunes 11 does not display embedded cover arts

Before I upgraded to iTunes 11 I prepared my whole music library and fixed all missing cover art. I have additionally embedded all my cover art (600x600px) into the ID3-Tag/File (TuneInsructor) and everything worked very well untill I upgraded to iTunes 11. At first glance everything looked well, but then I discovered missing cover arts. I have the feeling that the number of missing covers increases over the time. When I checked the song detail info of these albums/songs the the cover art was correctly displayed. And now we come to the strange issue - when I picked a album with missing cover art and chose e.g. the detail info of the first song and closed that window again iTunes correctly displayed the cover art again. Nevertheless this is an annoying behaviour of iTunes and I am not sure that it will not happen again - its's no fun to fix it all by hand over and over again.

I have to say, that I store my complete iTunes library and music files in my LAN on a Network Storage (NAS of QNAP). Here some screenshots to clarify the problem (don't be confused, it's the german GUI):

Does anyone has similar problems and/or has a solution to automaticly fix the problem ?

Happy new year 2013!

Togart