DNA isolation from BCG

[Mick]

Few weeks ago I started PCR test detecion of  Mycobacterium
tuberculosis  in tuberculosis patients,
but in electrophoresis I got only bands of positiv control.
I  have investigate about 20 samples obtained from patients with
 suspipitons of pulmonary and extra-pulmonary tuberculosis 
(sputum, pleural effusion, bronchaial washing).
I suspect there must be sth wrong with isolation protocol or
concentration of BCG.
I wil be grateful for any help.

 Michal Fik