It could be anything but try trashing the preferences to start with.
Many weird things happen as a result of corrupt preferences which can create a vast range of different symptoms, so whenever FCP X stops working properly in any way, trashing the preferences should be the first thing you do using this free app.
Shut down FCP X, open PreferenceManager and in the window that appears:-
1. Ensure that only FCP X is selected.
2. Click Trash
The job is done instantly and you can re-open FCP X.
There is absolutely no danger in trashing preferences and you can do it as often as you like.
The preferences are kept separately from FCP X and if there aren't any when FCP X opens it automatically creates new ones . . . instantly.