have more tools to use in this case and I can suggest you use add ons in Firefox to download videos from youtube.com. it's very efficient and you can try to search "youtube video download" in firefox addons box search.
The thing with YouTube downloaders is that they are not consistent. They usually have so many issues. You can choose between an online downloader or just having an App. I used to use the Download YouTube purple app, but it just stopped working. Right now if I want to download any YouTube videos I go online and use safeFrom.net. For now this works perfectly. It also downloads really fast and in any format that you require.