Why aren't all of my photos from my Photos Library showing in my Photos Library in iMovie?

Photos in the cloud will not show up if you have Optimized Storage selected in your Photos app preferences. Could this possibly be your situation?

Also, your Photos library must be designated as your system library in the Photos app general preferences.

-- Rich

Yes, because the original photo is in the cloud, not in your Photos library. Only a display thumbnail resides in your library. If you turn off the optimized storage and select the Download Originals to this Mac option the originals will download to your Photos library and be accessible to iMovie.

-- Rich

"Photos in the cloud will not show up if you have Optimized Storage selected in your Photos app preferences."

Is this true?

Go to the Photos app preferences. You may want to post in the Photos for Mac Forum for a more in depth commentary.

— Rich

As a workaround try exporting from the Photos app to your desk top, and from there dragging into iMovie.

— Rich