A 6-year-old girl with a history of gastroesophageal reflux is hospitalized with a primary lung abscess in the right upper lobe, diagnosed by chest CT. She is started on intravenous ampicillin-sulbactam. After 9 days of appropriate antimicrobial therapy, she remains febrile with persistent cough and chest pain, and a repeat CT shows no change in the abscess size. What is the most appropriate next step in management?