Lets get to point,
suppose this is the url of youtube video you are watching.
and you want to download it.
Your downloader, suppose IDM is not detecting the video.
what i suggest,
replace http://www. with ss
your url should be
enter this url in addressbar, this will redirect you to savefrom.net which helps in downloading files from other servers.
now you have choice in video quality, select quality and download video.