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try the face detection dnn, it's much more versatile wrt. pose:

#include "opencv2/opencv.hpp"
#include "opencv2/dnn.hpp"

using namespace cv;
using namespace std;

void drawPred(int classId, float conf, int left, int top, int right, int bottom, Mat& frame)
{
    rectangle(frame, Point(left, top), Point(right, bottom), Scalar(0, 255, 0));

    std::string label = format("%.2f %d", conf, classId);

    int baseLine;
    Size labelSize = getTextSize(label, FONT_HERSHEY_SIMPLEX, 0.5, 1, &baseLine);

    top = max(top, labelSize.height);
    rectangle(frame, Point(left, top - labelSize.height),
              Point(left + labelSize.width, top + baseLine), Scalar::all(255), FILLED);
    putText(frame, label, Point(left, top), FONT_HERSHEY_SIMPLEX, 0.5, Scalar());
}

int main(int argc, char** argv)
{
    float confThreshold = 0.25;

    dnn::Net net = dnn::readNetFromCaffe(
        "c:/data/mdl/face_detector.prototxt", 
        "c:/data/mdl/res10_300x300_ssd_iter_140000_fp16.caffemodel");

    VideoCapture cap(0);
    while(1) {
        Mat f;
        cap.read(f);

        Mat blob = dnn::blobFromImage(f, 1, Size(128,96), Scalar(104, 177, 123, 0), false, false);
        net.setInput(blob);
        Mat res = net.forward("detection_out");

        Mat faces(res.size[2],res.size[3], CV_32F, res.ptr<float>());
        //cout << res.size << " " << faces.size() << endl;
        //cout << faces << endl;
        for (int i=0; i<faces.rows; i++)
        {
            float *data = faces.ptr<float>(i);
            float confidence = data[2];
            if (confidence > confThreshold)
            {
                int left = (int)(data[3] * f.cols);
                int top = (int)(data[4] * f.rows);
                int right = (int)(data[5] * f.cols);
                int bottom = (int)(data[6] * f.rows);
                int classId = (int)(data[1]) - 1;  // Skip 0th background class id.
                drawPred(classId, confidence, left, top, right, bottom, f);
                cout << classId<< " " << confidence<< " " << left<< " " << top<< " " << right<< " " << bottom<< endl;
            }
        }
        imshow("F",f);
        int k = waitKey(19);
        if (k>0) break;

    }
    return 0;
}