I have always one nostril blocked (not completely thought) and according to "Kundalini Tantra" book that I read it's usual state for most people. In the book it was connected to the activity of brain laterals (one is dominant) and forcing yourself to breath equally through both nostrils will make both halves more balanced. Not sure if the explanations were correct, but the practice itself provided me with some really high states. Breathing is extremely powerful tool.