iPad to Mac - Album backup stops - appears to be on a thumbnail showing as grey box

I am trying to backup all my iPad albums to my Mac. Many of the albums back up fine. However during some of the backups, I get this error:

"There was an error communicating with the device.
Please make sure that you are using the latest version of
Photo Transfer App on this device."

I am on the same network, latest version etc. I tried to figure out what is making it stop. It appears that in some albums, there are some thumbnails that just show as a grey box. For example there is one called "daughter exercising.jpg". I can see this photo on the iPad. However, when I click on the grey thumbnail of that named picture in Photo Transfer App, it just says "loading..." and doesn't show the picture.

What could be causing this problem? I don't want to loose these pictures. Even if I try to download that 1 picture, then I get the same error message above. The picture is there on the iPad, but the App cannot see the image, only the name...
Most of the pictures transfer fine. I have already transferred 1,000s, but there are few hundred that aren't transferring.

Help!