McDonald's SA - Milkshake cannot be swopped

<p>Ordered a Chicken Sharebag via the drive through and was told that I could not change my drinks to milkshakes. I always make this change, mostly online. While complaining about this the person through advised that the machines were not working. With a heavy heart I completed my order without milkshake. Why can the staff not be professional when advising clients or try to ask the manager if items cannot be swopped out. </p>