problem with move_base

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I am trying to control a bot with a lidar. When i run the launch files to switch on the lidar and obtain the cmd_vel values from rosserial based on odometry data, the cmd_vel is being published by the move_base.launch file. But, when i run the navigation stack which uses this data, I get an error that says-" THe bot cannot move forward". A little debugging tells me that the navigation stack subscribes to cmd_vel from move_base. But, the cmd_vel is not being output by the launch files itself. How to solve this problem?q