How can I force the Photos app on my iPhone to recognize faces
Solution 1:
The only way I know to force it perform face recognition is to open Photos, Lock the iPhone, plug it to the charger, and let it locked and plugged for some hours.
Solution 2:
I find this occurs with photos received via iMessage and other apps. I use the duplicate function to duplicate the photo. The Photo app is then able to recognise that there is a face in the duplicated photo.