Adopt a hostile tone; demand from them a commitment to be on your committee, then you have a "bid" going into the admissions process. If they say they can't evaluate your profile, ask them how they can possibly handle evaluating their student's work. If they ignore you/stop responding, flood their in-boxes with follow-ups, CC'ing the department chair, graduate director, and dean of the college (perhaps even the student directory as well). They will admire your perseverance, tenacity, and boldness.