playlist not showing all songs

My son has one playlist of 2000+ songs on his iPhone. We updated his system today to 5.0, and he came to me this evening complaining that several songs are missing from the playlist. It turns out that the songs are actually present on the iPhone and can be found using the search function, etc., but when he tries to access them via the Artists button -- his standard method -- songs, and sometimes whole albums, are not listed.

I then created on the computer a single new playlist and copied over all the songs from the first playlist. The new playlist showed up fine, and some of the missing songs reappeared, but not all. A separate playlist just for a single "missing" artist also showed up fine together with all of the associated songs.

Any ideas for getting all songs to properly show up in his playlist, short of creating individual playlists for each artist?

Thanks!