How much free storage is on your iPhone? If the iPhone is low on storage, you may want to reduce the size of your iCloud Photos Library. Even with plenty of iCloud Storage you should keep the Photos Library small enough to fit on your iPhone as well, at least optimized. Do you have a computer as well, where you could archive photos that you do not need on your iPhone at all times?
You may also see the message, if the re is a problem with the internet connection. Try to connect to a different Wi-Fi, if Photos cannot load the images, to see, if the network is the problem.
Another possible reason may be the format of the image in question. Photos cannot process each and every image format. If you saved a photo shared with you or from the web, and then saved it to Photos on an older system version, the current system version may not be able to process it. To check for a format problem, try to download the photo in question using a web browser on a computer. Go to the site www.icloud.com, then sign in with your AppleiD, then open the Photos.app on this site. Look for the images that you cannot download on your iPhone. Are they still in iCloud? If yes, can you open them on your computer? There are two download options - the edited version and the unmodified original. Try to find out, if the edited version or the original are causing the problem. If the elite version is the problem, try to revert the photo in question to the original. If the original cannot be opened, use an external editor to convert it to a compatible format and replace the photo in iCloud by the converted version.
If the original is no longer in iCloud, you have to restore it from your backup.
