Is there a way to manually tag a person if facial recognition doesn't pick them up?

Yes.  You can zoom into single image view for the photo you want to tag.  The Mark Face button will then appear in the lower tool bar.  If you click on it, a rectangle will appear which you can place around the person's face.  Once you position the rectangle, you can then name the person in the text box that becomes available.

Alternatively, you can simply click and drag the People tag name to the image (or vice versa).  However, in this case, you will only see a generic icon in the People stacks, since no face has been identified.  (You could also use a keyword tag rather than a People tag for this purpose.)