Your mind has a way of trying to protect itself from traumatising memories. Sometimes people dissociate from traumatic experiences and may experience a lack of memory of certan details or times in their lives. This is a protective mechanism and your mind is just trying to keep you safe.
If you have a past trauma that is related to the problem you are seeking support for in hypnotherapy, then there is a good chance that will come up and if it is integral to helping you solve your current problem then you would hope it does come up. But if you have experienced trauma in an unrelated part of your life and you have already worked hard to dissolve the negative impact that trauma once had on your life, then your mind will likely see no reason in bringing that up again.
If you have experienced trauma but you are not ready to address it, then that is also your choice. Remember, you are in complete control during hypnosis. There is a misconception that once you are in hypnosis you don't have control about what you say. This isn't true. If it was, then I'm sure there are thousands of hypnotists in the world with loose morals who would happily ask for their client's bank account details and PIN and you would regularly hear horror stories in the news about it!
I am a certified DeTrauma Technique (TM) therapist which is an intensive hypnotherapy process that allows the client to heal from trauma without having to retell their trauma story to the therapist. Often having to talk about one's trauma can be retraumatising in itself, and this is a common reason many people refuse to seek help for their trauma using more
traditional methods of therapy. I have experienced first-hand when studying to become a clinical hypnotherapist, just how powerful this technique is when it was used on me. It's not a magic trick that will make you forget everything and suddenly feel better; it's the creation of a safe environment in your mind where you can process the trauma in a way that removes its power over you... life is too short to let the stains of the past trigger pain in your future. You can take back your power anytime you are ready to. I wish I had known about this therapeutic technique when I was a lot younger, but perhaps we discover things at a time when we are truly ready for them?