Video doesn't start...

I’ve been putting videos on my Sansa for a while now and all of them have been playing fine. However, for most of the new videos I’ve just converted, the videos won’t play itself at all.  When I click the video file a black screen will pop up and nothing else happens.  But when I hold the fast forward down I can see the video fast forwarding, which I thought was weird.  Can somebody help me with this please?