How to trigger download of thumbnails of icloud photo library to iOS?

Solution 1:

The only way I have found is to click through each one. I usually start at the top and swipe through them, and they should start downloading.

Solution 2:

Reactivating iCloud Foto library and immediately restarting the phone did the trick for me.

I deactivated iCloud Foto library and immediately activated it again. I then restarted the device (I killed the Foto app before deactivating and did not open it since then). After the restart, I opened the Foto app and charged my phone (make sure low power mode is off). I left it there for a couple of hours and suddenly, all thumbnails appeared. The same trick also works for keeping the originals if your phone won't download them until you open them.