Try trashing the preferences.
Corrupt preferences can create a vast range of different symptoms, so whenever FCP X stops working properly in any way, trashing the preferences should be one of the first things you do using this free app.
http://www.digitalrebellion.com/prefman/
Shut down FCP X, open PreferenceManager and in the window that appears:-
1. Ensure that FCP X is selected.
2. Click Trash
The job is done instantly and you can re-open FCP X.