camera calibration - unable to detect chessboard

I am using openCV's function to detect a chessboard, yet no chessboard is being detected.

The image I am using:

const Size chessboardDimensions = Size(4,8);
int main (int argv, char ** argc)
{
        frame = imread("/home/Georges/Desktop/a.jpg");
        cvtColor(frame, frame, CV_BGR2GRAY);
        int found = findChessboardCorners(frame, chessboardDimensions, foundPoints, CV_CALIB_CB_ADAPTIVE_THRESH | CV_CALIB_CB_NORMALIZE_IMAGE);

}

found always equals 0. Could someone explain me why?