why my tag dont always keep the new info?

It happens once a while that after tagging some files and updating my library in media player, the files tags change back to the old values. why?

I'd guess either failure of either

1 the Mp3tag tag write. Tag write failure was in the past sometimes not alerted by Mp3tag, and I don't know if this has changed.

2 The WMP library update. Then you made another edit in WMP and the old value was written back to the file.