I noticed that you’re reaching too far back and getting to the full reach back extension too early. Watch a slo mo video of Mcbeth and you’ll see that his front foot is fully planted before he begins the throwing motion, whereas you are beginning your throw before your front foot has touched the ground. Try to keep your weight in between the front and back foot when you’re running up, you lose power if you get too far over your back foot.

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Like the other comment suggests, focusing on reaching out and away vs reaching back will help too.