Wheels using ROS skid steer controller can't turn

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I've created a four-wheeled robot with skid steer controller which works with teleop_twist_keyboard node. However the robot can only move forward or backward and I can't get it to turn left or right.

The skidsteer controller:

<gazebo>
    <plugin name="skid_steer_drive_controller" filename="libgazebo_ros_skid_steer_drive.so">
          <updateRate>100.0</updateRate>
          <robotNamespace>/</robotNamespace>   
          <leftFrontJoint>left_front_wheel_joint</leftFrontJoint>
          <rightFrontJoint>right_front_wheel_joint</rightFrontJoint>
          <leftRearJoint>left_back_wheel_joint</leftRearJoint>
          <rightRearJoint>right_back_wheel_joint</rightRearJoint>
          <wheelSeparation>5</wheelSeparation>
          <wheelDiameter>0.6</wheelDiameter> 
          <robotBaseFrame>base_footprint</robotBaseFrame>  
          <torque>20</torque>
          <topicName>/cmd_vel</topicName>
          <broadcastTF>true</broadcastTF>
          <commandTopic>/cmd_vel</commandTopic>
          <odometryTopic>/odom</odometryTopic>
          <odometryFrame>/odom</odometryFrame>
          <covariance_x>0.0001</covariance_x>
          <covariance_y>0.0001</covariance_y>
          <covariance_yaw>0.0001</covariance_yaw>
    </plugin>
</gazebo>

<gazebo>
    <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
        <robotNamespace>/</robotNamespace>
    </plugin>
</gazebo>

Thanks.