Cavities on CT scan but no MAC in sputum cultures

Does anyone have a cavity (I have 2 small ones and 1 new one) and never tested positive for MAC? I've tested negative for fungus as well, only Haemophilous showed up earlier this year. I cough a lot, much mucus, even with the daily clearance of two 7% sessions & ACTs. My doctors at NJH have no answers and suggest a bronchoscopy. Since my cough and SOB are worsening, and with no other signs of an exasperation, I'm stumped.