One more thing . . . it may be nothing to do with it but give it a try . . . delete the iMovie Preferences and see whether that has any effect.
Deleting the Preferences.
If FCP X/iMovie start behaving unexpectedly in any way for no apparent reason, the problem could be caused by corrupt preference files.
The cure for this is to delete the preferences.
Hold down the Cmd and Option keys as you launch FCP X/iMovie and a window will appear asking if you wish to delete the preferences.
Click “Delete Preferences” and FCP X/iMovie will open in the default (as-new) mode with no projects etc. on the interface.
You will have to navigate to any project you were working on and reopen it.
Deleting preferences is completely safe and because of its simplicity it should be the first thing you do when troubleshooting.