Can't Play Videos on iPhone that have been uploaded to iCloud Photo Library.

I Have iCloud for all my photos and videos so they upload as soon as I take them. I wanted to go back and watch some old videos on my phone and they won't play- meaning they can't/aren't/won't download from my iCloud and play on my device. I try going into "edit" mode to get it to download that way and it downloads about a quarter of the way and theN stops. Sometimes it doesn't even do that and just tells me there's an error and can't download and to try again later....well I've been trying for about a week and still nothing.