The first thing to always try is to Quit FCP, Restart the Mac, Open your FCP project and try again.
Next:
THE FOLLOWING COMES FROM THE KEN STONE WEBSITE:
“Over 5,000 years ago Confucius wrote: ‘If you are toiling away, you have changed nothing and FCP heads South on you, [starts behaving in strange ways] then it is time to trash your FCP Preferences.’ ”
Click the following link for instructions.
https://www.kenstone.net/fcp_homepage/trashing_fcp_prefs.html
A great way to do this is to use “FCP Rescue” a free Apple Script that will Trash the Preferences for you (and restore nearly all of your user settings afterward).
There are versions for FCP (Pro) & FCE (Express) and a new one for FCP 5.
Download these free Apple Scripts at
https://fcprescue.andersholck.com/
or
https://pistolerapost2.com/fcprescue/
This is one FCP tip that has helped in solving hundreds of “odd” problems.