Why the /nav_bot/move_base can't connect to /nav_bot/map topic？

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Hi, all. It is well known that amcl_demo.launch in the package turtlebot_navigation is very useful for using turtlebot.

But I need to use it underneath namespace "softbot" and I added <group ns="softbot"> and </group> in the beginning and the end of amcl_demo.launch, respectively. Then it comes out error :

[ WARN] [1401200360.787114288]: Waiting on transform from base_footprint to map to become available before running costmap, tf error:

By checking in rqt_graph and using "rostopic info/echo" command, I found that the node "/softbot/move" couldn't connect to topics "/softbot/map" and "/odom". I have tried to use

rosrun topic_tools relay /softbot/map /map

or add

remap from="/softbot/map" to="/map"

when launching node move_base, but it failed.

Has someone encounted same problem? Thank you!