Direct publish joint position to KUKA iiwa 7 R800

Hi, I just started learning how to use ROS to control KUKA iiwa 7 R800. I followed all the instructions based on iiwa stack repo, and have successfully used ROSSmartservo app to connect my PC with iiwa robot arm via ROS. I can also subscribe position information from robot. However, when I tried to directly publish joint positions to iiwa, nothing happened. I checked nodes information and seems that iiwa has subscribed msgs but just not moved. What could be the possible reasons for this issue? Is there any additional driver or some settings needed if I want to do so?