rostopic echo does not work with Gazebo topics and my own topics

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Hi,

I met a strange problem, never seen before. We I try to echo a topic like /scan_raw from my robot, rostopic echo works fine.

Instead if I try to listen to Gazebo topics like /gazebo/model_states, it returns:

    WARNING: no messages received and simulated time is active.
Is /clock being published?

The same with a topic that I'm publishing with my node. I don't understand why. I'm pretty sure the topics are published since my node is based on the listening of /gazebo/model_states. So I suppose the problem is on the echo.

Thank you in advance.