First make sure the site streams through RTMP protocol. Some Flash vids are streamed through HTTP protocole (eg. Youtube) so much so that Rtmpexplorer or Rtmpsrv don't work.
But it also happens that Rtmpexplorer & Rtmpsrv don't work despite the RTMP protocole.
Eg:
http://videos.arte.tv/fr/videos#/tv/coverflow///1/120/
http://www.vosgestelevision.tv/index.php
If the authors of these wonderful tools could see why those bugs...