Some HEIC photos do not appear in iMovie

I don't know if this might be your situation, but photos that are in the cloud will not be recognized by iMovie. If, for example, you use iCloud Photos and have the optimized storage feature activated some photos may be stored in the cloud to save space, leaving only a thumbnail in the Photos library. The thumbnail would appear fine in the Photos app, but iMovie would not recognize it until the photo is downloaded. Not all photos would be stored in the cloud, but only some, that might explain why not all of your HEIC photos appear in iMovie.

-- Rich

To clarify, are you referring to some HEIC photos not being recognized in the Photos library item in the iMovie media sidebar? If so, are all the HEIC photos contained in the same Photos app library that is designated as the system photos library? Have you noticed any recurring difference in the metadata (resolution, size, etc) of the HEIC pics that appear and the one's that do not?

Try deleting/resetting preferences and see if that cures it. 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 library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

You can also try redownloading iMovie from your purchases folder in the online app store. First move your present iMovie app from the Applications folder to the trash, but don't empty the trash. You can restore it to the applications folder if the redownload doesn't work.

-- Rich