Instagram/Twitter only show a couple photos

I've got an iPhone 6, 16 GB, running iOS 8.4.

When trying to add photos to a Tweet using the Twitter app, or adding a photo to Instagram, I've found that the uploader only shows a couple of my photos, not all of them. When I look in my Photos app, I can see all photos in the Moments, Collections, and All Photos album, but not in the uploader for other apps. I have iCloud turned on for photos. These apps have permission to access my albums. Not sure what the problem might be. Thanks for reading.

Ray