I also use both Video Downloadhelper and youtube-dl to download videos from youtube and other sites and the majority of the time it works. However, some video are protected and sometimes plugins don't work and/or youtube-dl will complain with an error. Your only option, if you want the video badly, is to do a screencast recording of it.
Vokoscreen is a intuitive and simple screencapturing GUI program. It uses ffmpeg behind the scenes. You can record the whole desktop or just a portion of it, for example, the video window.
If you don't have vokoscreen in your repos, then search for screen capture in your package manager.
I saw this in another forum on downloading flash videos from the terminal using the /proc system to get the location of the video.
I haven't tried it yet, but it looks promising.