On an iPhone or iPad, you can't force a face ID if Photos doesn't recognize a that a face is there. If it recognizes a face, you can identify it, but if it doesn't see the face at all, you can't fix that.
Many of us try to include people's names in the Caption field, even if the face is already identified. Then a search for Charley, for instance, will find that picture of Charlie, anyway. Also, I open Charlie's People album, and I put "Charlie" in every caption, there, so a search for Charlie will find all his pictures, even if they weren't recognized. So I use the People album to help me add the captions.
A big advantage to using Captions for people's names is that Captions are included (if you choose) when sending a picture, but face IDs are not. Of course, the special advantage of Face ID is that in a group of people you can see exactly who is who.