How to get ALL videos/pics showing in iMovie?

With iCloud Photos and "optimize Mac storage" selected, recently opened files will be downloaded into the Photos app on your Mac, and the rest will be stored in the Cloud to save space on your Mac. The combination of files that are in the Cloud and on your Mac is variable depending on your usage. iMovie won't read or access the files stored in the Cloud. So, if the Photos playlist that you are working from contains files that have not been opened recently, it is possible that they are in the Cloud and not accessible from iMovie. You can try downloading them to the Photos app and see if that cures it.

Personally, rather than use Optimize Storage you would be better off purchasing an external drive, formatting it Mac OS Extended (Journaled) and storing your Photos app library on it instead of your internal drive. Then you won't have to worry about optimizing storage.

-- Rich

Personally, rather than use Optimize Storage you would be better off purchasing an external drive,

This will work with a desktop Mac, that we are not carrying around. Then it is no hardship to have an external volume with the system Photos Library connected at all times, but it might be highly inconvenient to have always have an external volume connected to a MacBook Air. The volume with the System Photos Library needs to be connected at all times, so the background processes can access it, even if we are currently not working with Photos at all. It is bad enough to have to wait for Time Machine to finish, before we can unplug the Time Machine volume, when we need to take our portable Mac with us, when we leave home. It would be worse, if we have to wait for Photos as well to close the library.

Rather than moving the System photos Library to an external volume, I would keep it small, so it can stay on the system volume and archive all other photos, that do not need to stay in the System Photos Library.

That makes sense, léonie. I wasn't thinking about portability, since I never carry my laptop around, but store on my iPhone the photos that I want to carry. I didn't realize that the system photo library external drive needed to be connected to the Mac at all times, although mine is. Portability aside, I think that one is better off with storing the Photos system library on an external drive, versus using optimize storage. I never liked the optimize storage system concept. I agree with your sound advice to keep the system Photos library small. Following your advice in previous posts, I now create a new Photos library for each year. That keeps them pretty small, as you say, and easy to update on an external drive.

-- Rich

Unfortunately, this is a consumer website, so we don't have testing equipment or the ability to remotely access your Mac to identify the cause of the problem. The techs at Apple Support do have remote access capability. You might consider calling them and see if they have any suggestion as to what is causing the issue. I think that optimized storage is involved in the cause, but could be wrong.

I have found that sometimes the software won't give you what you want, for whatever mysterious reason. It can take hours of time to try to find out why. Or, one can try a workaround. In your case you might drag or export the videos/pics that you want to use to a Finder folder on your desktop, and from there drag them into iMovie. Alternatively, you might be able to drag them in directly from Photos to iMovie.

Regrettably, I can't claim to be smarter than any other person. 😊

-- Rich