Robot doesn't move after 2d-nav-goal in RViz navigation tutorial

I've been following http://emanual.robotis.com/docs/en/platform/turtlebot3/navigation/#navigation this tutorial , i already have maps. I set the initial pose , the estimation is visibly good, then after i set the 2d nav goal, the robot do nothing. I have checked the topic /move_base_simple/goal is updated every time i set the 2d-nav-goal , but still the robot doesn't move. The cmd_vel is only published by the node move_base, but the robot doesn't move. I use Turtlebot3 burger, ros kinetic and ubuntu 16.04 , i modified the mechanic of the robot , i add spacer and make the LIDAR in higher position (around 35 cm).