iPhone not syncing old iCloud photos Our iPhone doesn't sync with old iCloud photos.  It only syncs the past 5 years or so.  How can you force a full sync?   My devices have a lot of space I believe.
I would check the actual space settings on your phone. iCloud can show how much space might be needed. 



Also, I would turn save space option to see if that makes the device more willing to sync down at least all the thumbnails needed.