adaptive images doesn't actually change the image src. The code gets the src for the original image, checks if a smaller image is needed, if yes and it doesn't exist yet, it generates it and then returns that image. What it means is the html will then load the new image believing it is loading the original => no src is needed. What you need to do is check the size of the loaded image to see if it's the original or the resized one.
Also remember that it works with screen size not with browser size so resizing your browser won't do anything, you have to actually try it on a smaller device.