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.

lol IMHO well wmp gets packaged on every single windows machine out there so people really get stuck with it don't they. And for the ignorant, they wouldn't know any better. lol How many other media players automatically connect to the net the moment you open it without any choice. Let's not forget What you should know about WMA Files

I dont use or share any wma files and ive changed the settings so it doesnt connect to the internet ever.
I only use it to preveiw stuff before i burn it cd or do whatever i want to do with it.
It does what i need it to do but im not a fan of microsoft products so what media player would you recomend?