don't know why can't select (change) the subtitle when i played DV

don't know why can't select (change) the subtitle when i played DVD.
even i tried to ...
1. select the subtitle by clicking the menu from the DVD.
2. See Windows Media Player Help. To access Help, start Windows Media Player
and then press F1.
3. press 'Play', then click'lyrics, title and subtitle', select the right
one.
....but it doesn't work. without any response.
my window media player's version is 11.
my computer is microsoft windows XP, professional version 2002 service pack3.
no error message, just without any response, it still played without any
subtitle even i select the traditional chinese or others.
i tried to play the DVD in my DVD player, it worked. i can select the
subtitle. just can't select it from my computer (window media player)
thx!