All photos are not backing up

How do you know it's not backing up to icloud? Do you mean it's not syncing to icloud?

Read here Set up and use iCloud Photos - Apple Support and if you're using that it's not part of the icloud backup What does iCloud back up? - Apple Support

If you are syncing photos to iCloud, Try Settings/Your Name/iCloud and slide off syncing. Wait a few minutes and then slide syncing on. Allow time for syncing to occur.

Last resort - have you tried signing out of iCloud and then sign back in?