I have a few pictures where the photo app has detected no faces at all.
Is there a way to force the app to rescan those picture for faces?
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I have noticed on El Capitan and newer (Catalina, Big Sur, Monterey) Photos that manually adding faces on individual photos (cmd+i, click circle with + , position manually and resize circle, type person name) then skipping forward one by one (right arrow or slide) will rescan sequential photos.
Then going directly to Faces and clicking person face which you were working on will suggest that more photos could be added.
This made Photos to gain additional 10-20% face recognition for me.
It seems, that if Photos didn't found any faces, then ... well ... it won't find anymore no matter how many times you will scan it. Photos does not try to find a face similar to those you already have in your library, but just searches for something that looks in shape like face and then asks you if they are in fact faces.
Some more info: https://discussions.apple.com/thread/7056659