Hi,
The data on your post indicates that you are using Mac OS 10.12, Sierra. If possibly you are recording your photos or videos in the HEIF or HEVC formats, you might have display problems. You would need to upgrade to High Sierra. Take a look at this Apple support link:
Using HEIF or HEVC media on Apple devices - Apple Support
If the above is not your situation, try using the free download, Handbrake, to convert your video to the Mp4/AAC format.
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