trimming videos on quicktime player

A few days ago I first used the command + Y option to trim videos so that I could edit out filler parts I don’t need without having to separate them into different videos. However, I tried to do the same thing today but each time i try the command it just makes the little “knock” sound when a command is unable to be used. I don’t know what I’m doing differently today that’s stopping it from working. Anyone have any idea how to fix this?