When Photos first loads an image, it scans the picture to see if there's anything in there that it thinks might be a face. If it finds what it thinks is a face, it records the identifying data, and compares that to the face data of all the other pictures, and hunts for a match. It never does that again. If it hasn't see a face, then you use stuff in the Info Window (⌘-i) to add the identifier. Perhaps you know all this, but here's how it works:
In the info window, below the Keywords field, is the Face ID field, and in there is a little + sign in a circle:

When you click on that, a little circle appears in the center of the picture, and you can resize it and move it to the face you want to record:

Then you enter the name, like Charley or Prince of Wales, or whatever. Then, if Charley has no People album, one will be created and this picture will go there. Later, if you need to change the name for some reason, you can do that in the People album, maybe making it King Charley III, or something.
The one problem we sometimes have is that the circle refuses to move, because Photos thinks you're still in the info window and want to drag the picture there, or something like that. In that case you need to tap on the picture to let Photos know you mean the picture, and then you can move the circle. Or, you can just type the name first, and then move the circle. I haven't done this since Sequoia, so this glitch might have been fixed.
Does this work for you?