When I edit music info it is not saved.

I've formated my mac a few weeks ago.

I've imported my lybrary with Time Machine and it works perfectly.

The only problem is that I can not edit any song's info. Nor title, artist, lyric, front cover..nothing! When I press OK all the new data I've written disappears and old information is shown again.

The files are not protected, I've already checked it.

Any help?