I've run into this problem a few times lately. I type a youtube url into smtube and it reports "no results found" even though the video definitely exists, because I can download the video with youtube-dl just fine. This morning I was using smtube 16.1.0 and entered the following url, in three variations, getting the same result each time. I successfully downloaded the video with youtube-dl.
https://www.youtube.com/watch?v=3JmhKwC77Xs
http://www.youtube.com/watch?v=3JmhKwC77Xs
3JmhKwC77Xs
No results found