Can a person's susceptibility to hypnosis be altered?
There is no a single answer for this, but I will tell from what I have read of Milton Erickson's ideas.

Everyone is suceptible to hypnosis. The difficult in being hypnoized depends on the person's experience more than a specific part of the brain, and this implies that the most important thing is how the hipnotist is capable of capturing the person's unconscious attention, which is purely determined by the person's experience.

In conclusion, the person's suggestability is completely alterable. In my experience (not too much), if you fail hypnotizing someone and you do not handle ir properly, the next time that person will be less suggestable. This is only an example, there are many other factors that influence this process.