Custom Costmap Plugin not subscribing to topic

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I am running Turtlebot 2 and trying to create a new costmap layer as explained here http://wiki.ros.org/costmap_2d/Tutori... . I want to subscribe to a topic and get data from one of my sensors. I have added the line to subscribe to that topic in the OnInitiliaze() function. But due to some reason, the callback function is not being called. When I run rostopic info my_topic, I find that a node has subscribed to it, but still my callback is not being called. I have also confirmed that messages are publishing properly over that topic. Any ideas what could be the possible reason for this ? Thanks.