PSE 2020 Facial Recognition

If you know which photos they are, you can select each one in the Media grid in full screen view (double-click the image). The Mark Face button will appear and you can mark the face individually.  Each face will then appear in the Named People stack.

You can also select multiple photos of the same person in the grid and drag the People name tag to the images.  In this case, the person will show up in their name stack, but if you open the stack, only a generic icon will appear.  This is because the face is not associated with the person via face recognition.

The most drastic (and not recommended) option is to start all over again by resetting the face analysis in Preferences.  You will lose all prior face recognition if you do this.