People not available to select when personalising lock screen

I have my iPhone set up to automatically detect people in photos I take. When I select the option to personalise the lock screen using my photos there is an option to cycle through photos of people that the photos app has detected. However, not all of the people who have been detected in the photos app are available to select. For some reason, one of my children is missing - they're not there in the list to choose to appear on the lock screen. If I go into the photos app they're there and clicking on them brings up all the photos they're featured in. So photos is working correctly - it has found them and the photos they're in. It just doesn't seem to be pulling the person through to the lock screen to be available to select. Any ideas as to why this is and a fix? Was happening on my XR but just upgraded to a 14 Pro and it's happening on this new phone too.