A few things:
1) Always when the application appears to not be working correctly, deleting preferences is worth a try. It is often all that is needed to fix weird problems.
To delete preferences, hold down command and option while starting FCP X and click Delete Preferences.
The application will start in a default state, and open an "Untitled" library.
Reopen the library you were working on, set the preferences the way you like them, and try again.
2) Do you have Chrome installed on this machine.
Chrome and its keystone daemons are known to hog the system videotoolbox and cause massive problems with video applications like FCP.
Read more here: https://chromeisbad.com
Once you get rid of them, you need to trash FCP and download a new copy from the App Store.