Itunes often stores modifications of tags only in its database and does not embed the information in the files.
I think there is a way somewhere in the iTunes community that allows to extract the covers from the thumbnail collection of iTunes but this is rather tedious.
It would easier to use the web-sources that can be found in Mp3tag (there is even one for iTunes) and tag again. [WS] iTunes Music, Music Videos, Movies, TV Shows
And for the future: iTunes is no tagger, always use Mp3tag 