A 38-year-old woman with asthma is prescribed albuterol (salbutamol) 2 puffs every 4 hours. After 3 weeks of regular use, she reports that the same dose now provides less relief of bronchospasm, requiring use every 2–3 hours to maintain symptom control. Her peak flow measurements confirm reduced bronchodilation. The albuterol inhaler is functioning normally, and she has good inhaler technique. Which receptor-level mechanism best explains this loss of drug efficacy?