I can't speak from personal experience since my experience has been almost polar opposite where I sometimes end up seeing entities on sub-breakthrough doses even... BUT...
I've been exploring looking at entities from the same perspective that Carl Jung had near the end of his career, which is more or less the same perspective that Plato had in regard to "Daimons" -- which were considered to be beings that existed in the soul (greek: Psyche) and were intermediaries between Spirit and Matter -- the world of Gods and the world of humans. Essentially Jung believed that manifestations of archetypes, alien encounters, mystical experiences, etc... Were not subjective nor objective, but both -- they had objective existence outside of our subjective selves BUT when they manifested it would be through a form or appearance that fit the subject. So whether someone sees the virgin Mary or an alien with a spaceship was just a matter of which psyche was perceiving the archetypal force manifesting out of the collective unconscious.
OK now.. how do I think this relates to your post? (Sorry for the longwinded pre-explanation)... Jung noticed that for some individuals archetypes would not appear as manifestations in dreams or visions, but rather as projections on to the space around them. This would happen when an archetype is deeper repressed than is typical... So as an example if someone is EXTREMELY timid and had repressed all their aggressive tendencies then they may never get to see a Mars-like (warrior) archetype in their dreams, visions, trips, imagination... Instead they would start seeing the world as more violent and aggressive, they would start to perceive things as more aggressive than they actually are.
Am I implying that perhaps there are certain archetypes/daimons that tend to manifest as DMT entities which are repressed in you? Not exactly, I don't know you well enough to say that. I'm instead proposing a possible theory, and it is up to you to determine if it sounds like an avenue worth exploring or not.
IF YOU DO choose to explore this possibility... My personal recommendation would be to learn how to lucid dream and use that as a spring board to make a journey through your personal unconscious into the collective. Typically you would start with "easy to reach" archetypes, like your Shadow and Anima/Animus. Once you develop a good relationship with those two, integrate them into your own whole self, then they should become very powerful allies in navigating the INFINITELY LARGE sea of the collective unconscious.
To be 100% honest... I would recommend this type of deep work on the self to anyone and everyone, not just to people trying to figure out why there's no DMT entities in their DMT trips... HOWEVER please be warned: the deeper you go, the quicker you will face things that will deeply disturb you. If these things were not repulsive/terrifying/disturbing then they wouldn't be repressed in the first place.
There are very real dangers and Jung witnessed many inexplicable things in his study of the collective unconscious, things that seem mystical and impossible, things that don't stay limited to the mind but alter our own body and the physical reality around us.
If you choose to take such a journey then the advice of "integrate, integrate, integrate" becomes ever more vital. You would quite literally be opening up your own personal pandora's box.