Q: Photo previews not being generated for some raw files

Sporadically, photos uploaded via my iPad from the camera connection kit, no longer preview on any device (Mac, iPad, icloud.com, etc.) A significant number show a little grey cloud and when clicked won't preview anything.

All of these images were imported and viewable fine by the iPad until it then uploaded all to icloud, which is when this issue started. It seems icloud is having issues generating preview files breaking Photos across all these platforms.