Cannot favourite some photos in Photos app on iPhone

It wouldn't be surprising if "From My Mac" photos were resistant to being favorited, just as they are resistant to being deleted by the phone. They are in a special storage area that really doesn't "belong" to Photos; it can only be managed from an external computer.

Can you characterize what other sorts of photos have this problem? That was the first category that came to mind, but any photo that is not really part of the Photos library might also be resistant to being favorited.

I don't use it, but I've seen reports that photos that arrive through WhatApp may also be stored in a separate area and have the same problem.

Can you characterize which photos have this problem? Could it be photos that have been pushed onto the phone by cable from a Mac (typically shown under "From My Mac")? I don't/can't have any of those to test.

What markwmsn said.

Also, try Share>Save to Files and then re-importing the video from Files. You would then have 3 copies-- 2 in Photos and 1 in Files-- so you can delete the two you don't want.