Gazebo contact sensor added to a robot link in urdf file is not working as expected?

I have added a contact sensor to one of the finger links of a robot's gripper as follows:bh_finger_13_link_collision 5 On lauching the urdf file, I am getting the `gazebo topics /gazebo/default/wam/bh_finger_13_link/my_contact` and `/gazebo/default/wam/bh_finger_13_link/my_contact/contacts`. However, on echoing these topics, I am not getting the collision data. In order to debug this issue, I have also added a contact sensor to a box model sdf, and made the finger link to touch this object. On echoing the topic associated with the contact sensor of the box model, I am getting the collision of the box with the finger link, but on echoing the topic associated with the finger link, I am not getting the same collision information. I am not able to figure out the reason behind this discrepancy. Any help is much appreciated.