gmapping with turtlebot2 simulation does not create map

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Hello:

I am running ros indigo 32 on the Nootrix virtual machine. I am trying to make a map and navigate it with a turtlebot2 simulated robot, according to this tutorial.

In a terminal, I run roslaunch turtlebot_gazebo turtlebot_world.launch

In another terminal, I run roslaunch turtlebot_gazebo gmapping_demo.launch. It runs, but after a while, I get this message: [ WARN] [1437734908.414750431, 97.100000000]: MessageFilter [target=odom ]: Dropped 100.00% of messages so far. Please turn the [ros.gmapping.message_notifier] rosconsole logger to DEBUG for more information.

In a third terminal, I run roslaunch turtlebot_rviz_launchers view_navigation.launch. Rviz opens, and in Global Options, the fixed frame is set to map. The map element of rviz appears in yellow, with a "No map received" message.

If I open another terminal with roslaunch kobuki_keyop keyop.launch, the robot moves both in Gazebo and rviz, but in rviz there is no trace of the readings of the sensors; if I try to store the map with rosrun map_server map_saver -f mymap, I get a "Waiting for the map".

I have checked the tf tree with rosrun tf view_frames, and the tree seems to be completely connected (I am a newbie so it seems I cannot attach an image yet); at least the tree has a map -> odom -> basefootprint -> base_link path on it.I have made this source source /opt/ros/indigo/setup.bash in every terminal and before every roslaunch.

I have been googling and looking for help, but i can't find where the problem is. I would really appreciate any help you may offer.

Thanks a lot in advance, Ana+