Might Be Copy protection, but to be honest I don't know!
I had to STOP wmp9 playing my DVDs on XP by telling it to only play then on WinDVD.
Which OS are you one, because if it's XP you can tell it to play them on WMP.
Go to My Computer.
Right Click the DVD drive -> Properties.
Choose the Autoplay Tab
From the Drop Down menu choose DVD MOvie
Then Highlight WMP Click apply then close