Photos won't sync from Mac onto iPhone

Hi - I have iOS 15.0.2 on my iPhone XR and macOS Big Sur 11.6 on my Mac. I normally have the last year of photos showing on my iPhone. But it's refusing to syncing properly.

When Syncing my iPhone from Finder on the Mac, I have the button ticked under Photos that says "Automatically include photos from Last Year". Normally this works fine. No photos will sync across.

I have updated both my iPhone and Mac softwares to the current version and restarted both devices.

As an aside - it will let me import photos from my iPhone to my Mac and all of my music is fully synced as usual (although with an error message about one song, which I've never seen before, but won't lose sleep over!)

Grateful for any advice. Thank you