iCloud photo library is not downloading all images to my Mac or iPhone

I set up iCloud Photo library when it was released to the public, and it has worked very well, apart from the fact that there are about 100 images (out of my library of 5k) that do not download to Photos on either my MacBook Pro or my iPhone 6. have an issue where, for some reason, a fairly large number of photos in my iCloud Photo library are displaying as blank with a small cloud symbol instead of the image. I assumed it was still downloading the files in the background, however, this has been the case for weeks and there is no activity indicated. I have restarted my iPhone, my iPad and my Mac, and done all sorts of other troubleshooting ideas, but they seem to refuse to download. Help!I have all the latest updates installed.

They are dotted throughout my library and do not seem to have anything in common. I assumed it was still downloading the files in the background, however, this has been the case for weeks and there is no activity indicated. I have restarted my iPhone, my iPad and my Mac, and done all sorts of other troubleshooting ideas, but they seem to refuse to download. When I click on one to see it larger it does not come back with an error, but just shows an empty pane.