I don't create FLV so I am not 100% sure about this (you might ask in the Flash forum) but I **think** that a "raw" FLV file needs some kind of support files to allow it to play... such as an imbedded player on a web site and/or a further conversion (with Flash?) to create a file that plays in a "standalone" fashion
But, as I said, you might ask in the Flash forum for more information