To fully stretch the hamstrings in an RDL, follow these two steps (and avoid one thing).
Step 1. Focus on pushing the hips back and letting the pelvis rotate.
Step 2. Once your hips stop going back, contract the rectus femoris (quadricep that crosses the hip) like you are trying to pull your thigh to your stomach.
DO NOT elevate the toes. This does nothing to stretch the hamstring. It only stretches the gastrocnemius (calves) which are needed to stabilize the knee. It does not increase tension in the hamstrings at all.