[support] FlashVideo :The thumbnail is not created.

Hello, I installed a Drupal multi-site, my site 'sites/default' and a second site 'sites/mysite2'. I use the FlasVideo to insert videos into my content. When I add the video content of my site, the thumbnail is created and if I click the video plays. But when I did the same thing since my second site, the thumbnail is not created and therefore my content I get the message: Video currently not available An idea? Thank you