Only Some Photos Videos Appear in iMovie

Clips in the cloud will not be shown in the Photos item in the iMovie sidebar.

If you use iCloud Photos and have the optimized storage option enabled then some of your videos and photos will be stored in the cloud and not recognized by iMovie. They will be recognized if you down load them.

The procedure for drag and drop is to export the video clip from the Photos app to your desktop and from there drag into iMovie. One used to be able to direct drag from the Photos app to iMovie but posters here have reported that that no longer works. I wouldn’t call it a design flaw, however.

— Rich

The four clips that don't show up are probably in the cloud. With optimized storage the Mac randomly picks clips that have not been played to store in the cloud, in order to save space, and will substitute small sized thumbnails to display in the Photos library, and on your devices that access the cloud. iMovie is compatible to read full sized photos in the Photos app library, but doesn't access a thumbnail that is simply a reference pointer. If you download the clip into your photos library iMovie should be able to read it. If iMovie doesn't read it after downloading the clip to your Photos app library, then possibly resetting/deleting iMovie preferences would resolve it. Open iMovie while holding down the Option and Command keys, and delete preferences from the box that appears. iMovie will open in a new library, so you would need to open your current library to access your projects.

I'm not familiar with the Photo Stream feature, having never used it. You might post your question on the Photos for Mac Forum. The contributors there are very savvy re the Photos app.

-- Rich