How can I add a missing person to my iPhone wallpaper from shuffle?

I have had my iPhone wallpaper set with the photo shuffle option and et it up to show photos of my wife and daughters. A couple of months ago I noticed that one of my daughters no longer appeared in the shuffle and when I checked, she was no longer listed under the person tab - the only people showing were me, my wife and my other daughter. I have updated the iOS numerous times, tried giving people names in the photo app, spoken to staff in my local Apple Store, yet nothing (and no-one) has been able to sort out why the photos were taken off and why only 3 people are showing in the options. My Apple Store actually suggested I reach out to the community, so here I am seeking your collective knowledge.

[Re-Titled by Moderator]

Original Title: My photo shuffle wallpaper has randomly removed a person from the shuffle. How can I add them back?