Problem with map generation in Rviz

[edoardo-cioffi-1997]

I am trying to geneate a map in rviz. My mobile robot mounts two lasers and I've used the ira_laser_tools to merge the two scans of the lasers. Problem is that after moving around with teleop, this is the map I get:

Clearly, there is something wrong. This what the map should look like:

What can I do?