This is a myth that is perpetrated by Hypnotists and Hypnotherapists to make their businesses bigger and more profitable but it belies the reality of the situation. About 95% of the population can be hypnotised to some degree which leaves about 5% who are totally unable to reach a trance state no matter how much they try.
Research has shown that there are many levels of hypnotic trance and that the more susceptible you are to the hypnosis the greater the impact of hypnotherapy. Some individuals are only affected at a very superficial level and for these people hypnotherapy can take considerably longer than those who have the ability to reach a deeper level of trance.
Although many hypnotherapists and hypnotist will disagree, the level of hypnosis has little or nothing to do with the quality and abilities of the hypnotist and everything to do with the capability of the client.
The ability to enter a deep hypnotic state lies with the subject and not the hypnotist and no matter how enthusiastic and prepared a client is, if they fall into the 5% they will never be hypnotised and hypnotherapy will have no effect on them whatsoever.
Not everyone can be helped through hypnosis and it is down to the practitioner to be honest with their clients to ensure that they offer the best service they can to those who can be helped, and honest advice to those who cannot be helped.