Quite recently, youtube had make a new update and some videos cannot be downloaded using internet download manager anymore.
to overcome this, you guys can use this mozilla firefox extension called Greasemonkey.
1.first you need to have mozilla firefox as a browser
download from here: http://www.mozilla.org/en-US/firefox/new/
2. install greasemonkey
https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/
3. install greasemonkey youtube download script from here:
http://userscripts.org/scripts/show/25105
( press install from the upper right corner of the page)
That's all. Good luck in doing this ^^
