Discount problem when adding 2 tens of same product

Hi.
I was trying out this plugin in order to future use, and I find it very good. I’m only having one issue, and I will try explain it below (sorry but English is not my native language).
I created a coupon to be activated only for specific state and for the minimum amount of $500,00. The state condition is working fine; the minimum amount condition too, except if I add to cart an item below $500,00 and increase it to two units (or more) of same product from cart page.
I created a test product which costs $499,00. If I add it to cart, I have no discount, and ok, should work like this. But if I stay in cart’s page and increase the number of products to two (or any other number > 2), the discount is not applied. But if I increase to a higher number (lets say 3 for example) and then decrease to 2, then the discount is applied.
However, if I clean cart, go back to store, add this $499 product, check the cart, go back to store and add this same item ($499), and go back to cart (instead of increase the number of items from cart), the discount is applied correctly.
How to solve that?

Thanks in advance.
Regards