I have seen this in a single person, who always blacked out on DMT, or had zero effects.
Remember that receptors are proteins with a complex folded structure, they vary in everyone and in some cases are more different than others, there are people who do not respond, or respond differently than others to molecules because of this.
It is a lot like allergies in some ways, some people just metabolize things differently.
Consider also that in some very rare cases this type of thing happens with psilocin and or mushrooms, where people black out, and in very few cases have died.
I recall a pharmaceutical company scientist observing that statistically speaking substances which are harmless for most people can be dangerous for a minority, this is also consistent for data for pretty much any drug or molecule. The scientist proposed that rumors of people dying on normal doses of both legal and illegal drugs could be legitimate and due to variation in receptors and metabolism, he cautioned that we should not ignore reports of people who have distinctly different effects, apparently in drug studies this is well known, a drug might work well for most and yet for one or two people out of hundreds of thousands it could be ineffective or harmful.
I don't mean to alarm anyone, statistically speaking DMT is harmless, but for a very small percentage of the population it can be ineffective or cause effects that are undesirable, much like pretty much any food or beverage can.
In my experience if people have no effects or unusual effects, it is not wise for them to continue experimenting with whatever is involved, increasing the dose can be a bit risky in this regard.
