Make sure you are using a dual link dvi cable. For some reason some 120hz monitors don't come with the cable that can actually handle 120hz. Also from my experience you need to change the refresh rate from the video driver settings not windows display settings.
edit: I just realized it comes with an HDMI cable and I'm fairly certain that can handle 120hz signals but I'm not sure. So assuming it can you need to go through your video card settings to change the refresh rate. If you have an AMD card it will look like this.