You probably have this set up on the standard bryce infinite plane with nothing else around it. If it reflects the things behind your perspective/camera what are you going to see? From this angle you should see the chair and lap of the person doing the drinking. You need to either build props or load pictures to give the glass something to reflect. Since it's an umbrella drink you could get a beach picture and put it behind the camera to reflect off the glass. Search the threads for the word hdri. These are image spheres. They help a lot when doing glass pictures.