Make sure its updated and loads on startup.Some sites like youtube have tough DRM & can prevent a video from being able to download.If its up to date,loads on startup,& doesn't download a vid.Try to download a random vid on youtube or something instead of a specific vid.Maybe the ones you are trying had a DRM update or something.Otherwise,uninstall it(revo uninstaller,or ccleaner recommended for uninstalls),reinstall,make sure it's updated,restart after its done.And try again.