Photos won't download from iCloud over cellular

I have iCloud Photo Library turned on and to optimize device storage. This is a great concept and allows me to store all my pictures rather than having to delete. However once the photos are stored in the cloud and only the low-res versions stored on my device, I am unable to download any of the full versions from the cloud unless I am on wifi. The download clock in the lower right gets 1/4 of the way and will stop, never to advance from there. I have checked all the settings I can think of and have found nothing. Please help!! Very frustrating!!!!