Scepticism isn't the same as unwilling to believe. Usually just honesty, willingness to engage in dialogue, answer questions etc, will do it.
However, I have had experience working with Americans in the UK that sounds similar to what you might have experienced. In my experience, the "selling" of the mission has usually just been excessive enthusiasm on the American side with a hope that it will be contagious. To us, it all appears just a little fake.
Despite being a fake (no one found a actual newspaper reference [0]) it obviously resonated very well. And still resonates.