Having one nostril blocked at a time is often due to the normal nasal cycle, but if it feels persistent or bothersome it may be related to Deviated nasal septum or Allergic rhinitis. You can try steam inhalation, saline nasal sprays, and avoiding dust or allergens for relief. If the blockage is continuous, affects breathing, or causes headache or sinus issues, consult an ENT specialist for proper evaluation and treatment.