I know how you feel. It happened to me in my last career, also on Type II. Found a Large merchant, fired only one torpedo at the bow. I was so delighted when her bow disappeared below the surface, and when I saw the lifeboats around her. Then - nothing.
The water was shallow, and while the bow was on the bottom, her stern was still above the surface. Then I fired my last two torpedoes hoping that she will sink. She didn't.
