Add on qty is not adding correct amount

Hi, my cart is not adding the reduced-rate shipping amount for additional items in the cart.
It adds the full shipping amount again instead of adding the small additional cost.
This happens when I increase the quantity of an item and when I add another same-size/cost item to the cart.

I can send you login.

Thanks

https://www.ads-software.com/plugins/table-rate-shipping-for-woocommerce/