Sounds like its because Windows Media player doesnt have a compatible DVD Decoder (which is one of the bas parts about WMP). I know Nero has a DVD Decoder, and some trial versions of DVD players should also include some compatible decoders.
do a google search for k lite codec pack. It contains codecs to play most any video file as well as a dvd decoder. I also suggest using a different media player like media player classic or vlc.