Hi,
Occasionally iMovie will cause an issue that is confined to only one project, and does not appear in other projects.
The first thing to try is to delete/reset preferences. To delete/reset 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. 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.
If deleting preferences does not cure it, then copy your media into a newly created project and try importing your troublesome clips into the new project. To copy your media into a new project while still retaining the edits that you made, do an Edit/Select All, Edit/Copy. Then open the new project and do an Edit/Paste of the media into the timeline of the new project.
-- Rich