Some iMovie projects don't copy

You might try deleting preferences and see if that cures it. Is it possible that you have insufficient space on your destination drive for the project to be moved or copied over?

To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie usually will open in a new empty library. If so, 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.

-- Rich

Have you tried redownloading iMovie? Drag your iMovie app from the applications folder into the trash, but do not empty the trash. Then go to the app store and update iMovie from the purchased list in your account. If no success, drag your iMovie app out of the trash and put it back in your applications folder.

— Rich

Hmmm. You might want to consider calling the techs at Apple Support and describing your situation. They have the capability of remotely accessing your computer that we do not have on this forum. To contact Apple support, click on the Support item at the top of this forum's screen and navigate to a place where you can input your phone number.

-- Rich

Ah. Good work. I should have thought of creating an new library.

There might be some corruption in your old library, so might be a good idea to copy all of the projects over to the new library.

-- Rich