I think there are many ways in to give explanations for your dosage situation. Here's some of the things I'm thinking of, but it's nothing concrete. Just want to give you some new starting points, if you hadn't thought of the following yet.
In many situations psychology plays a key role. This includes mind-set and the influence on your psychology or mind of the setting, including what people are surrounding you.
For many people it is important to have someone around, if they really want to let themselves go in normal dose-ranges. I see this as one of the many natural self-defense systems someone can have: you don't want to damage your body in the process of focusing on your mind, perception, inner space, spirit, etc.
It might also be a trauma or other experiences that have not been worked through by the person, which blocks you from entering states of mind in which you will be confronted with your history. I think this can also take place as a form of protection. Let me emphasize, though, that I *don't* try to say that everyone works like this and everyone will always be protected from hard experiences. It is simply one of the possibilities that something in your mind keeps you from psychonavigating, for whatever reason.
Also, your diet has a lot of influence on what a certain plant or compound triggers. There are many food philosophies around. You could dive into the ones you like and after trying things out you will have an ideosyncratic, you-based diet.
One established shamanistic diet, designed for undergoing plant-induced experiences, is based on unprocessed, organic, plant-based foods. You could do this for 3 days to a week before ingesting mushrooms, ayahuasca, LSD or another psychedelic or psychoactive compound. If you want to eat some fish or meat during this week, it's not a problem, but stick to a small amount on only 1 or 2 days, else the effect starts to wear of. Then one day during the diet take a compound on an empty stomach in a 'normal' dose and see if your sensitivity has increased. This is one of the ways to explore where your low sensitivity comes from.
Don't take it too seriously though. For example, John Lily has done very nice experiments with proteine based diets, involving >80% fish, meat and nuts. Basically, find something that you like and that gives you the feeling that it frees up body resources, so there is more energy available for your brain, rather than your digestive system. Some foods simply cost a lot of energy to digest and this varies from person to person in a lot of cases too.
Diet also works on the long run and influences your hormonal balance. I'm not an expert, but serotonin is an important hormone in the effects of psychedelics and everything consciousness and perception related. To make serotonin, you need certain nutrients (I think in the case of serotonin, tryptophan is one of the precursors we get from food like bananas). And to keep the part of the body that produces the serotonin happy, it needs a lot more than just the chemical precursors.
You probably have thought of several or most of these things yourself though, so I'm not sure if it is at all helpful. I just know by experience and from talking to a lot of psychonauts, several ayahuasqueros and reading the common books that your diet can be extremely influential on the outcome of a psychedelic endeavor.