SolvePnPRansac always returns an identity rotation matrix

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I am using OpenCV 3.1 and I am trying to estimate the rotation between two frames of a camera. First, I detect some keypoints, define their descriptors and match them. Finally, I have to see the rotation with solvePnPRansac (supposing all points lay on the z=0 plane), but it does always return the identity matrix as the rotation even though the image is rotated. Here is the part of the code that does not seem to work: (Get_Euler_angles is a function I made and works:

  std::vector< DMatch > good_matches;

  for( int i = 0; i < descriptors_object.rows; i++ )
  { 

     if( matches[i].distance <= factor*min_dist )

        { good_matches.push_back( matches[i]); }

  } 

  std::vector<Point3f> obj3;

  std::vector<Point2f> scene2;

  Point3f aux3;

  Point2f aux2;


  for( size_t i = 0; i < good_matches.size(); i++ )

  {

    //-- Get the keypoints from the good matches

    aux3.x=keypoints_object[ good_matches[i].queryIdx ].pt.x;

    aux3.y=keypoints_object[ good_matches[i].queryIdx ].pt.y;

    aux3.z=0;

    obj3.push_back(aux3);

    aux2.x=keypoints_object[ good_matches[i].queryIdx ].pt.x;

    aux2.y=keypoints_object[ good_matches[i].queryIdx ].pt.y;

    scene2.push_back(aux2);

  }  

  Mat rvec, tvec, rmat;

  Mat cameraMatrix = (Mat_<float>(3,3) << 374.67,0,320.5,0,374.67,180.5,0,0,1);

  solvePnPRansac( obj3, scene2, cameraMatrix, noArray(), rvec, tvec);

  Rodrigues(rvec, rmat);


  std::vector<double> euler_angles (3);

  std::cout << "rvec from solvePnPRansac: " << std::endl << rvec << std::endl;

  std::cout << "rmat from solvePnPRansac: " << std::endl << rmat << std::endl;

  euler_angles=get_Euler_angles(rmat);

  std::cout << "euler angles from solvePnPRansac: " << std::endl << "[" << euler_angles[0] << " "<< euler_angles[1] << " " << euler_angles[2]  << "]" << std::endl;

  std::cout << "tvec  from solvePnPRansac: "  << std::endl << tvec  << std::endl;

If I use two images as input I get the matches and a translation vector, but rotation vector is empty:

rvec from solvePnPRansac: 

[0;

 0;

 0]

rmat from solvePnPRansac: 

[1, 0, 0;

 0, 1, 0;

 0, 0, 1]

euler angles from solvePnPRansac: 

[-0 0 0]

tvec  from solvePnPRansac: 

[-320.5000258050974;

 -180.4999954623273;

 374.6697010743459]