Photos can only use the faces it has detected itself to learn what a person looks like.
So you have to name the faces that Photos has automatically detected to improve the face recognition. The faces circles you are drawing manually around a face will not help to teach Photos how the person looks like.
You may not yet have named all faces that Photos has detected.
To find the faces that need naming, I would enable the checkmark "Show faces names" in the "View" menu. This way you will see a circle around each automatically detected face, when you are viewing a photo enlarged.
You can also use a smart album (File > new Smart Album" ) with the rule "Person is ..." and leave the name empty.
You will find all automatically detected faces in this album, where the name is still empty.
This way you will also find the detected faces of people, that have not yet been added to the People album and do not appear in the "Add additional Faces" dialog.