Try converting your .mov clips to H.264, Mp4/AAC with the free download, Handbrake. Handbrake will do it by default. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures the issue.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich