A 35-year-old male construction worker from Delhi presents with a 3-week history of productive cough, low-grade fever, and night sweats. Chest X-ray shows a cavitary lesion in the right upper lobe. Sputum smear microscopy is positive for acid-fast bacilli (AFB). He is started on anti-TB therapy under RNTCP. After 2 months of treatment, his sputum smear remains positive. Which of the following is the most likely reason for this treatment failure?