What format are the troublesome videos? What device were they imported from? What steps did you use to import the video clips? Do they play in the media browser but just not the timeline? You can try importing the clips to the Photos app and then accessing them from the Photos item in the sidebar of an iMovie project. See if that works. It is the recommended way of doing it.
Possibly a corrupt preference is causing the problem. Try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if the troublesome videos will import to the timeline.
Also, the issue might be confined to just one project. Try creating a new project and dragging in the videos into its timeline. See if that works.
Possibly the videos have not been fully imported. That can happen if the camera or SD card is disconnected or unplugged prematurely. If that might be your situation, try re-importing the videos.
-- Rich