I have been an Extraordinary Minister of Holy Communion for just a few months now. I am very surprised by the number of people who don't say amen. Often, I will hesitate in handing them the Eucharist or the cup and then I will hear a belated amen.
So apparently they know what to say and are not saying it. Very strange.
Good idea to wait. I always look them in the eye too. At least that was part of the instructions given to us by our priest.