robot not following path in rviz

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I tried to follow up this tutorial given in http://moorerobots.com/2016/09/27/aut... and was able to load the launch file roslaunch mybot_gazebo mybot_world.launch, start map building by using the command roslaunch mybot_navigation amcl_demo.launch and launch rviz using this command roslaunch mybot_description mybot_rviz_amcl.launch. All this commands work fine and I was able to load the robot and when I give it a direction using 2D nav goal it is able to calculate the path but it doesn't follow it. What's wrong? Can anyone help me?