Hi DiniG,
There are many reasons why that could happen, but the most obvious one may be that the video is still loading, and you need to give it some time.
Also, have you got the latest version of Adobe Flash Player? You can download the latest version at
http://get.adobe.com/flashplayer/Updating may resolve the issues you are experiencing. If you have any further problems however, do let us know and we will look into the problem further!
Cheers,
shuki