I mean there isn't much to send... make sure something is actually happening on screen that can be recorded. I actually fell for that trap thinking it's not working, except I just recorded empty background and put a video of empty background on the empty background. Other issue could be that if you record with automatic, it can record either webm or mp4 depending on your system, so putting the source could end up in the wrong one (mp4 in the webm source), not sure if that would break it but it's possible.