A few photos (only a few) not syncing across devices ... why?

My photos are synced across my two Macs and iPhone. This works perfectly nearly all of the time, with photos taken on my iPhone appearing minutes later on my Macs, and photos loaded from my camera to one of my Macs appearing a few minutes later on the other Mac and on the iPhone. That's how it's supposed to work, I believe.

BUT, a week ago I scanned some old negatives (10 photos) in my scanner, and imported the jpegs to one of my Macs ... and they have NOT appeared on my other Mac or iPhone. Photos taken later (and earlier) all sync fine, but these ones do not sync. The "Info" data has the correct date of 19th April 2023.

Any ideas what has gone wrong? Why should everything else sync fine, but these photos refuse to sync?

Thanks,

Chris