Is it possible that you are using iCloud Photos and have the optimized storage option enabled? That might prevent iMovie from reading many of the photos in the Photos app library. iMovie won't read photos stored in the cloud.
The format of your external drive that stores the Photos library should be Mac OS Extended (Journaled). If you need to reformat be aware that reformatting will erase all data on the drive.
Also, try deleting preferences, as that sometimes cures iMovie misbehaviors. 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.
If still no luck, you can drag your photos directly from the Photos library into the timeline of an iMovie project. Or drag to your desktop and them from there into iMovie.
-- Rich