Can't alter info on songs

I have downloaded some songs for Itunes on my pc. Normally I can alter the info on them, change the artwork,ect. But there have been a few songs (mp3) format that won't allow any changes, even when it seems to take the change( like new artwork for example) it vanishes when I click ok. Is there some kind of encoding that doesn't allow changes to the info on these individual mp3s?



Thanks

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