Action server callback not working

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I'm implementing the action server below:

  class NavigationServer:

    def __init__(self, name):
        self._action_name = name
        self._as = actionlib.SimpleActionServer(self._action_name, rodain_get_directions.msg.NavigateAction, execute_cb=self.execute_cb, auto_start=False)
        self._as.start()

   def execute_cb(self):
      rospy.loginfo("starting callback")

For some reason my callback function is not being called and messages aren't written in log file. Any idea why it is happening. I have a launch file that starts both client and server sides and client works fine.