Couple of extra tips that might help:
Point your foot, knee and upper body in the same direction as the punch.
Push backwards on the foot to get directional energy.
Try holding the waist back a little (up to 45 degrees to the hip) and then releasing it to power the arm.
Vibrate the hip with the the wrist rotation for penetration. Use "hammer and screw" action of the wrist to get that "stabbing" effect.
Release the punch as the toes touch the floor and complete as the heel lands (forget the heel bit if it's staying raised).
Ensure the elbow points downward so you don't open the shoulder and lose power.
Squeeze the last 2 fingers tight to activate the larger group of forearm muscles to strengthen the wrist and protrude the first two knuckles. Ensure the webbing of the hand captures the tip of the index finger to stop it sliding out of the fist.
Soften immediately after the required penetration to be able to continue immediately.
Hope it helps...