There are probably many different reasons why some people are nonresponders. I don't think anyone is a nonresponder to all points in all applications. I seem to be a nonresponder to head strikes. I think I know why.
In my case, I think I am a nonresponder to head points because the people who think they can KO me really have a hard time psychologically to hitting me with intention. My coach can and he has come the closest. I went down once and came up seeing double for about 15 seconds. So reason #1 would be skill of the attacker and intent.
Second would be that people who don't respond to one point are more likely to respond to the next point down the (meridian) line. That is what Dillman says. I have no research to back that up.
The next reason would be that some people are more tolerant to the dizziness or resist going down for a "stage 1" or "stage 2" KO. You might need to put them "out" as in a stage 3. We don't practice that way. At least most of us don't. It is safer to just go ahead and let that person be a nonresponder.
Next there are anatomical differences such as the size of a foramen. These are holes in bone through which nerves and blood vessels emerge. A large foramen is easier to hit and would have a better effect. Other important differences might be the length of a ligament or how "cut" a person is since nerves and blood vessels can be found in the cracks between muscles.
Another problem is the skill and knowledge level of the attacker. Some points set up others. Fortunately, the body often moves in a way so that it tells you where to hit next. Opposite quadrants is a good way to go. There are other clues and guidelines.
Now this covers the "striking" applications. Remember that we also use points for grappling and tuite applications and take downs and so on. I think most of the time when people talk about nonresponders, they are talking about "striking". I know that as a "nonresponder", I surprise some people with my ability to resist the pain and the action of having my head turned via SJ 17, as an example, and there are others. Here the problem is that I have all my energy invested in resisting. I respond much better with a set up that takes my energy away from the particular point.
So yes, there are nonresponder situations for a variety of valid reasons. But then, OK move on. Do all your leg sweeps work? Do all your tuite work? Do you know someone with an iron wrist that just can not be manipulated? Is there someone you can't choke out?
No specialized type of offense is going to work in all cases. That is why we train a more complete martial art. For me, pressure points is PART of what I do. My Dan rank certificates do not say "Kyusho".
For me, I can learn something that I can use and want in anybodies dojo. I also believe that if you came to my dojo on Kyusho day, you could learn something that would improve what you do too.
The older I get, the better I was!