Later in your study you will begin moving before uke is able to grab. In a fight, for example, you wouldn't allow someone to clamp down on you unless you messed up. Katate dori is an exercise to explore exactly what your question suggests, "How do I control the quality of contact." Rather than asking uke to change, experience the technique with multiple partners. If they all let go then something needs to be altered. If some let go then something needs to be altered. Enjoy.