Why can't I fast forward songs in windows media player?

I've got some songs in my library that can't be fast forwarded. If I make an attempt, the song just starts back at the beginning. If I play them directly from their source, I'm able to ffw. I've tried removing and re-adding them but it doesn't work. I have the same songs in my itunes library and they work fine there, but I prefer using wmp. Any ideas on how to fix this?