If you are rendering and re rendering the same shot, especially if there are large still photos in the timeline the memory gets overwhelmed.
Go to your render file for the project and delete all the renders, remember that there may be more than one render file for a project.
Your timeline will now be unrendered, simply re render the whole timeline again.
Yes FCP is totally lame because it can’t use all the memory you have in your computer, just one more thing on a very long lame list.