"File format not supported" randomly appears

I just bought the Sandisk and used windows media player to populate the Sandisk 32G memory card.  My files are .wma and some play while others don’t? I get that same “file format not supported” and then a screen appears as if it were trying to play, but there is no progress across the bar and no sound.   It displays the correct album/song info, but shows a 0:00 length.  Has anyone else had this problem?

K.