This downloader is great, but I've recently encountered a bug.
There are sometimes multiple download buttons on a youtube video, and the number increases for each video I watch. I have to uninstall and reinstall the extension to get it back to normal, but it will start increasing again eventually.
This has been fixed, update to latest version.
I too have multiple buttons none of which work. Maybe the latest version of Youtube has broken this addon as it has not been updated for quite a while.