Perhaps it's more likely that you're not comfortable with the method.
Me, I just can't deal with suffocating in an exit bag (I tested), nor with the effects of partial hanging (I tested), and because there are other methods available to me, I can let those two go and move forward.
I chose another method, the best for me of all available options, but have a minor issue with tachycardia, and a major issue with suffocating, so shortness of breath scares me. I'm having to work with the fears by imagining the entire process of the method and how each potential symptom will feel, and how I will manage it. As a result, I feel less afraid of the act of dying, and closer to being ready.