I have used Shotcut in the last few months to edit product demo videos and it was working very nicely. Was using the Microsoft product before.
Today, I have a few more videos to edit and now it no longer works as before. I opened the video, ran it to where I need to cut a clip out. Paused it and hit “S”. Nothing happens. Click on the icon that looks like a page with a split line , and same thing nothing happens.
It is an MP4 from a Galaxy Smartphone.
Have you tried reinstalling? I have absolutely no idea if that would be helpful, and it would delete your custom settings and I just don’t know if that’s a good thing. You could do backups of course.
Personally I would do that almost by instinct in a situation like yours though.
Actually when this started today I was on 16.XX.
I downloaded the new version and it does the same thing.
Hmm. To my knowledge updating is not a full reinstall.It simply overwrites the changed files. Assumed. And wait for someone who knows something before you reinstall, but I wanted to share the option 
Ok, I will uninstall, then re-install.
False alarm!
Forgot to drag the video to the time line…
Oops!
Thanks for the help.