If I say "Ok Google", the assistant starts up just fine. However if my wife says it, nothing happens. She tried it several times, speaking loud and clear, but nothing happened. Is the assistant somehow expecting my voice only? Voice Match ist disabled, though.
Train her voice by going to
Settings → Google → Voice → Voice Match → OK Google Voice Model → Retrain Voice Model
on 7.1.2 for other versions you may need to search in settings