Hey, Downloading videos from youtube sometimes become a bit difficult for some people. In many cases, the downloading software that you are using fails to detect the URL of the video correctly. I have a solution for such cases. If you are using any version of the Mozilla Firefox, simply follow the process.
For Windows 7 users:
Open a video in youtube and let it buffer completely. Then go to your C drive(system drive)/users/your user profile (For ex:Ibrahim)/Appdata/Local/Mozilla/Firefox/profiles/qb89pwze.default/cache.
Now, you will find a number of files of different sizes. Right click your mouse and then go to "sort by" and select "size". By doing this, the biggest files will come on the top of the page. Now select the big files which are at least more than 1 MB in size and rename them with a .flv extension. Now open these files in vlc player or any player that can play .flv video formats such as windows media player classic, flv player etc.
This process requires a lot of time as it takes quite a long time for buffering. If your downloader is working properly, then its wonderful but if its not working, this process can be simply good as a temporary solution.