Update not working for decreasing

Hello. Thanks a lot for useful plugin.
I need to do minus on product.
whenever I try decreasing item count from product it says: The quantity for \”your_product\” has decreased to \”n\
But whenever I refresh the page, I see it wasn’t updated and it’s the same as before.

How I did this:
I determine the item key from backend and save it in a hidden tag. and use it to do ajax.
The ajax response calls my product name truly. But seems doesn’t do it’s functionality.

But whenever I try to decrease one by one. Whenever it reaches 0, The function works and it really remove item from cart.
Did I do something wrong?
Thanks