Could be a number of things that come to mind, but I'd have to know more about your hardware, software and or internet connection.
This is why I can't act as a tech support guy for these broadcasts. It's just too time consuming and often fruitless.
My best advice, if your determined to get it set-up right, is to visit the forum at Sopcast.org. They have people reporting problems of every variety, so you may find someone with your exact same concern.
I know this may sound weird, but did you have the media player (sopcast or another media player) sized to full screen?
I used Windows Media Player (there is a button on the Sopcast player to transfer the signal to WMP), and I got no video unless I clicked on the button to display WMP at full screen.
It also works the same way with me for the sopcast player (I only get video in full screen mode).
I have no idea why this happens, but I don't think it is isolated to my system.