Failed to add additional product to existing order

We are using opencart 1.5.6
In the past, our staff can add product to an existing customer order, as long as the shipping address etc field are filled in.
However now we have a case where the staff cannot add a product. The order has 7 items, among it all are free except one. When the staff adds a product to the order, the status said Please wait (in yellow bar), then a green bar saying order total has already been calculated. (Sorry our version is a Chinese version so I'm just interpreting what the messages say - so may not be exact wording)
We are puzzled by this phenomena. What could be the solution?

i have solved this problem myself, sorry for wasting your time. I noted the status of the product to be added is "STOP", so after re-enabling the product, I can add the product to existing order.
Thank you