Auto update cart after quantity change

Almost one year late, but this question might still get visitors:
You trigger the click, but the button doesn’t have enough time to become enabled, so that is why, by the time you click the second time the button becomes enabled. Remove the “disabled” propriety before triggering the click:

    <script>
    jQuery('div.woocommerce').on('change', '.qty', function(){
        jQuery("[name="update_cart"]").prop("disabled", false);
        jQuery("[name="update_cart"]").trigger("click"); 
    });
    </script>