Excellent advice from everyone - even you, Walter. (Just kidding)
Assuming the person wants to learn (and is not being pushed to do it), there are several key spots in training that some people will have difficulty with. Most of the time, assuming the equipment is working correctly, this just takes a little time and getting comfortable.
Anyone that has taught for a while has seen this, I'll bet Walter has seen hundreds. Most will get over it with a little practice, a few will not.
I've noticed that some people are initially bothered by the individual regulator design and everything from the mech. nature of the breathing to the sound of the bubbles. Only time I have seen people not get over this is when they really did not want to do it in the first place, but I have been told, every so often, that is not the case.