Hi James
The classic set-up for this pose is squeeze hips push them forwards, and bring arms and upper body back. In effect you are in a slight backbend. What many people do is move their bodies to one side without any backbending component. In other words it is a straight sideways movement. I wonder if this is happening for you. Creating some lift in the chest and a simultaneous backward movement of your arms may change this pose for you. Pay attention too to your shoulder alignment. I am assuming that you have great alignment but it doesn’t hurt for me to reiterate some essentials. 😉 Bring your arms back and rather than the lower shoulder forward, bring the upper shoulder back. This can help you with your chest lift. If it is none of these things then it could be your unique body geometry.
Namaste
Gabrielle 🙂