How to force photo thumbnail download on iOS 8.3

I have 30,000+ photos synced to iCloud and am trying to get their thumbnails to show on the Photos app on iOS 8.3. On the (first) Photos tab, viewing at Years or Collections levels shows lots of grey placeholders instead of photos. Only when I click into Moments do the thumbnails actually load.

However, trying to scroll through all my photos at the Moments level, to trigger downloading of the thumbnail, would take hours. (It's a little faster under Albums > All Photos, but not much.)

Is there a way to force all the thumbnails to download? I tried disabling iCloud Photo Library and re-enabling it, but it did not force a sync. Note that I have Optimize iPhone Storage set, but I thought this should still download thumbnails.

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.