When I edit metadata tags in the media library for .MP3 & .wma files I understood that the info was written to the file itself. However, when I play some tracks, the data I have entered is deleted. I haven't worked out any correlation between those files where this happens and doesn't happen. Specifically, what I like to do is put the year in the 'genre' tag so I can view tracks arranged by year.

Does anyone have any idea why this is happening and how/if I can stop it?