iTunes doesn't let me change the song's artist

I have a folder named after an original soundtrack. All the files in there were originally sorted by song number and title. Then I changed, in the folder, the tag to 'artist - songtitle', because many of them aren't from the same artist. However, when I play it in iTunes, still showed the right title but the artist said Soundtrack. It's really frustrating! I've tried to change it on info, on the itunes, and it does let me. But if I play it again it automatically changes back to 'artist: soundtrack'. What do I do? I don't know if I explained it right, but there are other files that I can easily change the artist name, but others don't. Like this one. Please help me.

Thanks a lot.