The problem is youtube blacklisted IP address ranges used by web hosting. So when script try to access video page, youtube show a captcha page instead of normal video page.

Youtube video download sites like KeepVid: Download and save any video from Youtube, Dailymotion, Metacafe, iFilm and more! stopped working.

One solution is using a proxy server, but getting working proxy server is not easy and youtube can ban proxy servers too. So even with proxy, there is no guarantee.