iCloud isn’t backing up all my photos

You are using the word "backup" for the iCloud syncing. Are you syncing the iPhone with iCloud Photos to have a backup (a safety copy of your photos)? If you enable iCloud Photos on the iPhone, the opposite will happen. You will no longer have a backup of your photos, because the photos in iCloud Photos (Library) will no longer be included in the iCloud Backup of the iPhone, and with "Optimize Storage" enabled the photos will be excluded from the iTunes backup of the iPhone as well. If you should accidentally delete a photo terminally from your iPhone, there will be no way to recover it, because it will have been removed from iCloud as well and there is no backup of the photos.

iCloud Photos is fine for syncing the photos between your devices, but if you are using iCloud for syncing your devices, you have to make regular backups of your photos by downloading them from iCloud to a computer and archiving them. The only way to have an iCloud backup of your photos is to make regular device backups of the iPhone to iCloud (About backups for iPhone, iPad and iPod touch – Apple Support) and not to use iCloud Photos, as the photos are excluded, when you use iCloud Photos.

See the small print on this page: What does iCloud back up? - Apple Support

When you use Messages in iCloud or turn on iCloud Photos, your content is automatically stored in iCloud. That means they're not included in your iCloud Backup.

You have started a syncing process where the photos you apparently thought you were just backing up, within limits, will end up being synced to all your devices such that photo syncing has been enabled. That's not just a simple backup process. And, the photos on every other enabled device will end up, again within limits, on the device that you though you were just backing up.

léonie is talking about a phone backup that can be restored directly to, for example, a new phone in setting the new phone up. You can to such backups to iCloud, but I would never do so because, for example, the photos on the phone are put on iCloud, but not on the backup. I don't like this idea so I always backup my phone up to a computer. In this regard, tho, you could do phone backups both ways at pretty much the same time if you want. Anyway, léonie posted this link: About backups for iPhone, iPad and iPod touch – Apple Support) Check out what it says about how to backup a phone to a computer.

I don't have a good answer. But, you could try something low-level, but standard. Try logging out and back in to iCloud.com. And, try shutting down and restarting your computer.

I think it is prudent to back your photos up if you are having an iCloud/photos syncing problem. Of course, you can back them up to an external drive or on your internal drive. If you use your internal, to cover all iCloud/iCloud Drive the safety bases, I would back them up to a new folder in your Home folder.

The iCloud backup will give you the benefit of an offsite storage. The plain iCloud backup of your iPhone will include everything and you can restore your iPhone from the backup, if it is lost or stolen or damaged. And if you are having several backups in iCloud, you can even revert to a previous version, if you accidentally delete all your photos. But this will no longer work, if you are using iCloud Photos in addition to the iCloud backup, because the iCloud backup will no longer include the photos and videos from your Photos Library. You will still be able to restore your photos from iCloud Photos Library, if the device gets lost, but there will be no longer any way to recover photos you accidentally delete or to protect you from user errors. You will have the one copy of your photos in iCloud Photos, but no history. One careless gesture and all your photos might be gone. The iCloud Photos Library has your working copies, but no archive. You must not make any mistakes, because anything you do with your photos on the iPhone will sync immediately to iCloud Photos. You will need the local backup to protect you from user errors. Or at least you must never, ever empty the Recently Deleted album to have some safety line. I would not risk to work with iCloud Photos without having a local backup. And the added bonus is, that I can access my photos, if there should be some network trouble that cannot be immediately fixed.

See the article https://support.apple.com/en-mt/HT203516

You're welcome 😊

Good Luck!