I am using vision.Cas​cadeObject​Detector to count no. of faces in an image, however it is not counting all the faces. Any help would be appreciated .

So, below is my code to count number of faces using vision.CascadeObjectDetector but it isn't counting all the faces in the image. Kindly help.

a=imread ('images.jpg');

imshow (a)

size (a)

face_detector= vision.CascadeObjectDetector();

bbox= step(face_detector, a);

b= insertObjectAnnotation( a,'rectangle', bbox, 'face');

imshow(b), title( 'detected faces' );

n= size(bbox,1);

str_n= num2str(n);

str= strcat( 'Number of faces are ', str_n);

display(str);