I do not use iTunes as a tagging program - that is left to MP3tag.
But in this forums, there have been a number of threads that describe the updating problem with usually iTunes as source.
iTunes apparently tends to store tag data only in its database but only reluctantly writes modifactions back to the files.
The advice I gathered so far: there seems to be a function in iTUnes that converts the tags from one version to another - so converting V2.3 to V2.3 (or whatever you find in your files) would write the database information into the file.
In respect to the updating iTunes: You have to force iTunes to update files it has already loaded. please look for iTunes scripts that do just that.
So check out this thread: /t/18721/1