maybe because u own a none-updeted Xbox , or your account is PAL ,
there are some items that u can find them in all market places
for ex. Japanese market place u will find more than 20 Jap Games demo
but if u search for them in the US market place u will not find them , what ever
Sign up for a new gamer tag , forget this gamer tag , I hope u dont have a good game score
because u will lose every thing ...
if u are using your old HDD, thats mean your gamer tag saved on it , delete it , sign up
for a new 1 and select US , and your problem will solve , See u on Xbox live soon ....Bye Bye