First, a little education. Song information (title, artist, album, year, artwork, etc.) for a song is stored within that song's file itself. Whenever you edit that information using iTunes or other software, the information is changed within the song file. In addition, iTunes keeps its own database of this information so that it doesn't need to constantly read it out of every song file in your library each time you launch iTunes.
Now the problem. On many people's systems, iTunes doesn't notice when the song files in its library are changed, and carries on using the old information in its database. Indeed, any outside program that edits song files is sometimes effectively ignored by iTunes. Anecdotal reports indicate that iTunes 4.8 and later seem to be even worse in this regard.
For many people, simply selecting and deselecting a song in iTunes is enough to get iTunes to "notice" the changes and update its own database.
For some MP3 files, iTunes appears to be much more stubborn, and refuses to pay any attention to changes in a song file, and conversely will not put modified song info into the song file, even if you do the editing within iTunes. In this case, iTunes is only updating the information in its own database. For those, the following steps seem to be the only way to get iTunes to start keeping in sync with the information stored within the MP3 files:
- Select the song(s) in iTunes.
- Select "Convert ID3 Tags..." from iTunes' Advanced menu.
- Change them to ID3 tag version "None". OK it.
- Do the same thing again, setting them to ID3 tag version 2.4.