Why are my photos not downloading from iCloud?

Just tobe sure - have you enabled "Optimize iPhone Storage" for your iCloud Photos?

Your 15725 photos and 1425 videos will also need storage on your iPhone. It will depend on the size of he individual image and video files. As an estimated - if you are having roughly 50GB of photos and videos in iCloud, the optimized versions will need perhaps 10% to 20% of the size in iCloud (5GB to 10GB) and may just fit onto your iPhone, but it is unlikely, that they will fit without optimization.

I would not keep more photos and videos in iCloud than will fit onto your iPhone with optimize Storage enabled and leave enough free storage for other apps and software updates.

I'd try a forced restart:

Force restart iPhone - Apple Support

This is different from a simple Power Off. I had to continue holding the button down past the "slide to power off" message, and then I released the button only after  the Apple Logo appeared indicating a re-start. 

In any case, it's important to understand that full storage on a phone ( or mac or PC or iCloud) means trouble-- there has to be room for files to get "handled" by the OS. The numbers you see might not be accurate.