Songs show up but won't play on MP3 player - Got songs from MSN mu

Anyone can help?

I bought two songs at MSN music service, and used Windows Media Player (10,
I think) to transfer the songs to my MP3 player. I can see the songs on my
MP3 player, but when I go play one of them, it just skips to the next song
that was already on my MP3 player (before downloading these 2 MSN music
service songs). I have been successfully playing 7 other songs that I
purchased from Napster. It's a Napster MP3 player that came with my Napster
service, and Napster said their system comes from MSN, so the songs should
play.

Thanks for help,

David