When i try to use the gpu ray sensor in my velodyne model urdf file, it is not publishing any topic. When i put the same line of code for sensor in another urdf file of another robot it works fine and i can also visualize the topic in rviz, but i cant use the same sensor in below urdf file, what is the problem can anyone once check it. Here when i launch it in gazebo i can clearly see the blue laser coming out from the model.

<?xml version="1.0"?>
<robot name="laser">

  <link name="base">

    <visual>
      <origin rpy="0 0 0" xyz="0 0 0.017335"/>
      <geometry>
        <mesh filename="package://myrobot1_description/meshes/velodyne_base.dae"/>
      </geometry>
    </visual>

    <collision>
      <origin rpy="0 0 0" xyz="0 0 0.029335"/>
      <geometry>
        <mesh filename="package://myrobot1_description/meshes/velodyne_base.dae"/>
      </geometry>
    </collision>

    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0.029335"/>
      <mass value="0.5"/>
      <inertia ixx="0.000090623" ixy="0.0" ixz="0.0" iyy="0.000090623" iyz="0.0" izz="0.000091036"/>
    </inertial>
  </link>

  <link name="top">
    <origin rpy="0 0 0" xyz="0 0 0.085455"/>

    <visual>
      <geometry>
        <mesh filename="package://myrobot1_description/meshes/velodyne_top.dae"/>
      </geometry>
    </visual>

    <collision>
      <geometry>
        <mesh filename="package://myrobot1_description/meshes/velodyne_top.dae"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.000090623" ixy="0.0" ixz="0.0" iyy="0.000090623" iyz="0.0" izz="0.000091036"/>
    </inertial>

  </link>

      <joint name="joint_1" type="revolute">
        <origin rpy="0 0 0" xyz="0 0 0.077"/>
        <axis xyz="0 0 1"/>
        <limit effort="1000.0" lower="-10000000000000000" upper="10000000000000000" velocity="0.5"/>
        <parent link="base"/>
        <child link="top"/>
      </joint>

      <gazebo reference="top">
        <sensor type="gpu_ray" name="head_hokuyo_sensor">
          <pose>0 0 0.025 1.5707 0 0</pose>
          <visualize>true</visualize>
          <update_rate>30</update_rate>
          <ray>
            <scan>
              <horizontal>
                <samples>30</samples>
                <resolution>0.01</resolution>
                <min_angle>-0.174</min_angle>
                <max_angle>0.173</max_angle>
              </horizontal>
            </scan>
            <range>
              <min>0.001</min>
              <max>100</max>
              <resolution>0.01</resolution>
            </range>
            <noise>
              <type>gaussian</type>
              <mean>0.0</mean>
              <stddev>0.01</stddev>
            </noise>
          </ray>
          <plugin name="gazebo_ros_head_hokuyo_controller" filename="libgazebo_ros_gpu_laser.so">
            <topicName>/velodyne/laser/scan</topicName>
            <frameName>camera_link</frameName>
          </plugin>
        </sensor>
      </gazebo>

    </robot>

And confusing thing is that when i run rostopic list i cant see the topic /velodyne/laser/scan but when i run the command rostopic echo /velodyne/laser/scan i can see the information, like header, range distance etc. but when i launch rviz and put the fixed fram as camera frame than laser topic as /velodyne/laser/scan i can see nothing.