I have saved your image and this c++ code finds the missing 4 faces as well as all the others (note the different Haar cascade and the minimum size):
Mat im1 = imread("JuVIA.png", CV_LOAD_IMAGE_COLOR);
vector<Rect> faces;
CascadeClassifier cascade( "C:/local/opencv249/sources/data/haarcascades/haarcascade_frontalface_alt2.xml" );
cascade.detectMultiScale( im1, faces, 1.1, 2, 0| CV_HAAR_SCALE_IMAGE, Size(30, 30) );
for( vector<Rect>::const_iterator r = faces.begin(); r != faces.end(); r++ )
cv::rectangle( im1, *r, CV_RGB(255,0,0) );
imshow("in", im1);
imwrite( "miss4.png", im1);
waitKey();