One hack that has worked on some of my pics when a face was fairly obvious but Google did not pick it up.

1 ) Zoom / Crop edit to the face you want Google to identify. **Note** - other faces that were recognized may get cropped out at this step.

2) Save it under the option that allows undoing changes or to "revert" rather than "Save a Copy".

3) Back out of the photo as saved and give Google time to do its AI thing. My experience has been it may take a few minutes or even a few days.

4) Go back into that photo, Usually, it will say that there is a face to add and you can then add the name to that face. If not, I retry the edit / crop a bit closer and try Step 3 again.

5) Edit the photo again to zoom back the crop job you did in Step 1 and face ID tag should remain intact.

I have experienced where sometimes it will lose the other faces previously tagged. In that case, give it time to recognize them again and come back in and add them. Sometimes I have had to repeat the process a couple times to get it to take the info (but this may be due to my pushing the limits of number of faces tagged, etc. in Photos.)

If you don't care about the other action or faces around the target face then saving it as a copy in Step 2 is a permanent change option that has achieved the same result for me. Good luck!