With all the help you have locating the point, I'd guess that you probably have it. The greater probability is that you are not striking with good technique. How many years in martial arts do you have and what is your rank?
Next, what would you do if you actually HIT it? Suppose you got a KO for example. Who has coached you on your revival techniques or what would you do with a stage 3 KO which would mean an uncontrolled fall by a person in a complete black out?
I'd say get a teacher and work under supervision only for this point. If you want to work pressure points without supervision, use points for grappling strategies. Then you wouldn't end up with an injured training partner or a medical situation that you couldn't handle.
Now I have no clue who you are, age, skill level or rank. I'm making the opperating assumption that you are young and relatively inexperienced. If that is NOT the case, accept my apology as this may sound disrespectful when in reality, I am trying to be helpful.
Instead of GB 20, try using SP 11 to assist opening the guard. That would be much safer.
If you still want to hit on the head, try St 5 or Mental foramen. Both points have clear easy to find holes in the skull where they are located. Hit St 5 down and in LIGHTLY like on yourself first to see if you can feel the buzz. Mental foramen is hit down and across. Just hit lightly to see if you can get a buzz. Do not hit for a KO without a teacher. If you can't get the buzz on these easier to find points, then the issue is striking technique. Don't hit harder, hit better. It doesn't take much.
The older I get, the better I was!