Guess what. Your face is not comfortable. She does not want to sit on it. Unless it's shaped like a recliner, sitting on your face is work, and that's exactly what she does not want to do. The only reason why she would want to sit on your face is because you have already given her an MBO (mind blowing orgasm) and she wants to reward you, or she believes you will never give her an MBO and therefore wants to smother you to death.