Thus in any demonstration, there's usually someone in the audience who is more than a 'non-believer': strongly opposed to psi, who has the effect of damping or negating the demonstrator.
The other thing that's rarely mentioned is that most psi phenomena manifest themselves in moments of extreme peril or emotion, which (by their very nature) cannot be replicated in a laboratory situation.
Campbell also pointed out that, throughout human history, psi has been misunderstood, and ultimately maligned by religion, that most contagious of memes. So it's a subconsciously suppressed ability, and its suppression (and repression) is the cause of many human ills...
He believed that psi is an emergent phenomenon (like consciousness and intelligence), and - foreshadowing Penrose's "The Emperor's New Mind" - that the explanation for psionic phenomena lay in quantum physics and information theory. I've highlighted those last few words, because current information theory does not adequately cover phenomena such as complexity, synergetics and emergence. For a taster of what a new extended information physics should look like, check out Tom Stonier's "Information And The Internal Structure Of The Universe".