My photos can’t download from iCloud

Then you may have an issue with your Internet connection at this time, unless there is an actual problem with iCloud Photos, which you can check for using the link below:

iCloud System Status  (US)  iCloud System Status (UK) 

If you are in a different location use the UK link, select the location flag in the bottom right corner of the screen, choose your country from those available and then click on the URL of the next page that loads to discover your country code (i.e https://www.apple.com/au/, Australia=au). Once you have your code go back to the UK system status page and substitute uk in the URL with your own country code.

Welcome to the Apple Community.

Are you trying to send them through a third party app? If so, try opening them in your photos at first, so that it forces you to download a full resolution version to your device.