Try deleting preferences and see if that solves it.
To delete preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen your old library to get back to your projects. (To reopen your old library close iMovie and go to your Movie folder or wherever you have your old iMovie library stored. Double click on the library’s icon. iMovie will open in that library.) Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
Another thing to try is to create a new project and copy your media into it, and then see if you can get a complete export from the new project. To copy your media and your previous edits into a new project, do an Edit/Select All, Edit/Copy, in the original project, and then go to the newly created project and do an Edit/Paste. You may first need to click on the timeline of the new project to make sure that the media will paste into the timeline.
-- Rich