A first morning sputum specimen is received for acid fast cu…

A first morning sputum specimen is received for acid fast culture. It is digested and decontaminated by the N-acetyl-L-cysteine alkali method. Two Sabouraud dextrose slants are incubated in the dark at 35OC with 5-10% CO2. The smears reveal acid-fast bacilli but the slants show no growth after 8 weeks.  What is the explaination for this?