Some of my photos aren't showing up in iMovie. How can I use all of my photos for making a movie?

Some of my photos won't show up in iMovie, when I can see them in the actual photos app. How can I get all my photos to appear in the media section? I have even given up on iMovie and downloaded another movie app, and the same thing happens, I still can't see some of the photos? How can I fix this?