disparity image is displayed but pointcloud shows only a small dot in rviz

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Gazebo Setup

• I faced the exact same issue while simulating a custom stereo camera in Gazebo.
  This happens if you don't set the hackBaseline parameter.

• Following is the Gazebo plugin I had used:

  <!-- SENSOR PLUGIN -->
  
  <gazebo reference="left_stereo_camera">
  
      <sensor type="multicamera" name="stereocamera">
          <always_on>true</always_on>
          <update_rate>10</update_rate>
          <camera name="left">
              <pose>0 0 0 0 0 0</pose>
              <horizontal_fov>${radians(front_camera_horizontal_fov)}</horizontal_fov>
              <image>
                  <width>640</width>
                  <height>480</height>
                  <format>RGB_INT8</format>
              </image>
              <clip>
                  <near>0.1</near>
                  <far>100</far>
              </clip>
          </camera>
          <camera name="right">
              <pose>0 -${front_camera_baseline} 0 0 0 0</pose>
              <horizontal_fov>${radians(front_camera_horizontal_fov)}</horizontal_fov>
              <image>
                  <width>640</width>
                  <height>480</height>
                  <format>RGB_INT8</format>
              </image>
              <clip>
                  <near>0.1</near>
                  <far>100</far>
              </clip>
          </camera>        
          <plugin name="stereo_camera_controller" filename="libgazebo_ros_multicamera.so">
              <robotNamespace>/</robotNamespace>
              <cameraName>stereocamera</cameraName>
              <alwaysOn>true</alwaysOn>
              <updateRate>10</updateRate>
              <imageTopicName>image_raw</imageTopicName>
              <cameraInfoTopicName>camera_info</cameraInfoTopicName>
              <frameName>stereo_optical</frameName>
              <hackBaseline>${front_camera_baseline}</hackBaseline>
              <distortion_k1>0.0</distortion_k1>
              <distortion_k2>0.0</distortion_k2>
              <distortion_k3>0.0</distortion_k3>
              <distortion_t1>0.0</distortion_t1>
              <distortion_t2>0.0</distortion_t2>
          </plugin>
      </sensor>
  </gazebo>
  

• If hackBaseline is set to 0.00:
  
  Here you can see the dot in RViz at the centre of the optical frame.

• If hackBaseline is set to 0.06:

References

• Test Stereo Camera in ROS
• Stereovision in ROS
• ROS Robotics Projects

Note: Please let me know if this solution doesn't work out for you. If in case it does, feel free to upvote my answer.