One distinct factor that is important for hypnosis to occur is an agreement to participate in the process. Someone can willfully resist hypnosis or they may have an inner part that is resisting the process. This is why education is so important at the outset to allay any fears or misconceptions about hypnosis and set those inner parts at ease to trust the process and the hypnotherapist/hypnotist. In some cases I may take a client right into Parts Therapy to address an inner part that has resistance to the process.
Beyond the need for education and making sure the client is fully on board is the skill of the hypnotherapist. Different people may need a different style of induction to get you into the hypnotic state and an adequate intake and initial conversation will help direct the hypnotist as to which path to take.
90%+ people should be easily taken into hypnosis by a skilled hypnotherapist assuming they don't have inner resistance or misconceptions about hypnosis. The other 10% (or less) may be on a prescription medication that makes it more difficult, or have a mental disability. All in all nearly everyone should be able to be taken into some depth of hypnosis. Hope that helps!