Have you ruled out a physical blockage?
If you have, this might be of use to you as a starting point:
Brain activity influences the pattern of our breathing
We know that there exists the so called nasal cycle which is influenced by our central nervous system and that it contains a period when we can breathe more easily through the left nostril and another analogous period for the right nostril. This is connected with changes in cerebral hemispheric dominance which was proved with EEG recordings showing that higher activity of the right brain hemisphere results in easier breathing through the left nostril and the higher activity of the left brain hemisphere results in easier breathing through the right nostril and that this cycle changes every 25 to 200 minutes (3). So when we breathe more easily through the left nostril, the functioning of the right brain hemisphere is more pronounced and dominant than the left. Easier breathing through the right nostril, on the other hand, means that the left brain hemisphere is currently dominating. When we can breathe equally through both nostrils there is some kind of balance between left and right brain hemisphere functions and throughout the nervous system. Interestingly this happens mainly in the time of dawn, midday and sunset when the vital energies of nature are at their strongest.
Can we influence the brain activity by practising pranayama?
But what is even more interesting is that, besides this influence of the brain on our breathing process, the influencing also works the other way around – in other words, we can use our breathing to influence the brain. By practising the alternating breathing through the left and right nostrils we can naturally balance the functioning of both brain hemispheres (11). Careful observation of our body and breathing can show us which part of our nervous system is more dominant in a given moment and enable us to act in accordance with it. By that we can, through conscious effort, influence the functions of different brain centres. We can also increase the perception of mental and physical energy and awaken feelings of alertness and enthusiasm, thus creating a positive mood (7). These discoveries are the results of many studies dealing with the selective stimulation of brain hemispheres through alternate nostril breathing (4)(5)(6)(7)(8)(9)(10).
Breathing through the left nostril stimulates the right brain hemisphere and intensifies the functions of the parasympathetic nervous system. This is connected with a slowing-down of the heart rate and consequently greater stroke volume of the heart (greater amount of blood is pressed out of the heart per beat, as a consequence of lower heart rate) (12).
Breathing through the right nostril stimulates the left brain hemisphere and the sympathetic nervous system. This results in accelerated heart rate, higher blood pressure, greater consumption of oxygen by the body cells and constriction of the blood vessels of the skin (13), thus preserving body temperature (which explains how some yogis can be totally oblivious to extreme weather conditions).
One study reports changes in the concentration of blood plasma catecholamines and their correlation with the nasal cycle (14). Catecholamines are hormones, which regulate many body functions and are secreted upon stimulation of the sympathetic nervous impulses. This happens in conditions of mental and physical stress (pain, cold, heat, oxygen deficiency, fear, anger). Catecholamines trigger the release of glucose and fats (as energy sources) that are stored within the body. They also cause constriction of the blood vessels and increased blood pressure, heart rate and strength of heart contractions. Additionally, they dilate the bronchi, increase the basal metabolism of the body and mental activity and promote the body's capability to coagulate blood. There are many more effects of sympathetic stimulation but let it suffice to mention the above most important ones. It has been frequently observed that people who suffer from some anatomical changes in the nose (deviated nasal septum, etc.) and breathe mainly through the right nostril, stimulate the sympathetic nervous system and consequently they have problems with high blood pressure. A simple remedy for this is a small operation of the nasal septum after which they can breath equally through the both nostrils and the levels of blood pressure soon normalise.
Alternate nostril breathing (as in nadi sodhan pranayama) balances the functioning of the left and right brain hemispheres (11), improves spatial orientation and spatial memory (left nostril), improves verbal expression (right nostril) and cognition (4)(6)(8)(15). The latter enables us to gain knowledge about the world around us, to analytically define it, to formulate memories and, based on our experiences, to plan and act accordingly. Among mentally handicapped children, a considerable improvement of cognitive functioning and social adaptation was achieved by practising pranayama, asanas and short meditations, for five hours a week. The improvement was clearly more pronounced in this group of children than in the control group in which children did not practise any yogic techniques (16). We mentioned that with the help of alternate nostril breathing it is possible to also influence the heartbeat. When we lower the frequency of the heartbeat (by practising exercises such as nadi sodhan pranayama) we allow our heart to rest a little bit longer than usual between two consecutive contractions. This additional resting allows the heart to be filled with larger quantities of blood and to contract with greater strength, thereby strengthening the heart muscle and maintaining it in better condition.