Global Costmap only publishing once to a subscriber?

Hello. I'm using the amcl_demo.launch file to localize to a static map, and gather costmap data of the environment. When I try to subscribe to the global_costmap/costmap topic, it only publishes once, and will not update the value. The same effect is seen when using rostopic echo. Rviz can update the global costmap display, so I don't know what the issue is. Thanks.