iMovie 10.0.1. No thumbnails

I have Mavericks and iMovie 10.0.1. My thumbnails come and go. Movies are there, but I cannot get the thumbnails to stick. Nor can I figure out how to select a frame from the Event or Project to represent the movie as its thumbnail. I have run Disk Utilities. I do not see that there is a way of launching iMovie (like iPhoto or Aperture) and perform maintenance drills. Does that exist? Is there command to copy and paste thumbnails?