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.’ ”
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.