Ive just had the same problem myself,it started as soon as i edited the tags.
I deleted the effected files then copied them back to my shared folder from where id backed them up,i then opened them up in wmp and edited the tags useing that and the problem was solved.
I took time to edit the tags because the tunes seem to get 10 times more hits when you do and i dont see the point of sharing music files that wont play on the worlds most popular media player regardless of what some people think of it.