If the Add Faces Button seems to be unresponsive, check, if you are in Edit Mode

Written for Photos 5 on macOS 10.15 Catalina:

The usual way to add a face, if Photos did not recognise a face in a photo is the following:

In Photos for Mac, you may want to enable the Faces circles to be always shown, as long as you are tagging faces.

• Go to the menu bar and enable "View > Show Face Names".
• Now you should be able to see the faces circles, when you are viewing a photo enlarged.
• Select a photo and open the Info with ⌘i. Hit the space bar to view the photo enlarged.
• The "Add Face" button will appear in the Info (below the keywords). The button is just showing a "+", and "Add Faces" will appear, when we hover above the "+". It is shown while viewing the enlarged. Clicking it will add an additional faces circle; we can drag it to the face we want to tag and resize it.

However, sometimes nothing will happen, if we click the +. The usual reason is, that we are in Edit mode. Then the "+" (Add Face) button is available, but the faces circles are not shown, even with "Show Face Names" enabled. Photos will just keep to add faces circles, every time we hit "+", but we will not see them.

• If no circle is appearing, check, if you are Edit Mode and stop trying to add a face. Hit "return" to leave the Edit mode. Then you should be seeing instantly any faces circle you add.

If you are using iCloud Photos and the photo or video needs downloading from iCloud, it may take a while, until the "Add Faces" button appears at all.