Pretty much, Photos doesn't look for faces in the same picture twice. It goes through and identifies things it thinks are faces, calls it "unnamed," and then it tries to identify them as a second step. It will never again find a face in a picture it's already looked at. If it didn't see someone the first time, waiting or prodding won't help.
The "confirm" thing is about faces that remain "unnamed;" it knows they're faces, it just isn't sure whose they are. There's no way you can get more "unnamed" faces for it to confirm, except to load in new pictures. Manually naming faces has no effect on Photos' face recognition.
I started that paragraph with "Pretty much," because, who knows for sure? Most of this is stuff we had to just figure out. You could try starting a new library, re-load your photographs, and maybe it will find more.
Most of us have decided to accept it for what it is. The main use of the face names is to help add keywords. For me, I find every picture with Mary Ann in it, and I add the keyword "MaryAnn" to all those pictures. Face recognition helps me find the pictures with Mary Ann, but there are more that I hunt for on my own. I have completely stopped manually adding little circles and putting in names--except for pictures like family reunions where thee are 5 people I don't know but want to remember and keep straight. Even in those cases, I'll add the names as keywords, and put the names in order in the caption.
Keywords and captions become part of the metadata of the picture, and they can be included when you send the picture to friends. They survive transfers to other libraries and OS overhauls. Face IDs and little circles are not standard and are not dependable.
