How to force photo thumbnail download on iOS 8.3
I am having a similar problem.  All of my iOS photos have synced up to Photos on the Mac, but photos that I've added on the Mac show up blank in iOS. When I click on a photo, a blank comes up, with a progress clock in the lower right corner. However, that seems to stall at 25%. Even when I go into Edit mode, it appears to start downloading and appears to give up after 25%.

I may try to turn off iCloud Photos and try again, but I have nearly 70k photos and videos.