Whenever I try to use Downloadhelper to download videos from the web, I only get the first 3-5 minutes of the video. When I try again, I get the next 3-5 minutes after that. Shorter videos download fine. What can I do to download the whole video at once, and not just little pieces?

And I'm behind the Great Firewall of China. In case that changes anything. Since Youtube is blocked, I usually get videos from 56.com or youku.com