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I will provide a general answer that could be helpful in this situation along with many other, similar situations.

If nothing is happening (or nothing is to be seen like in your case), first check rostopic list. If the topic that is causing you the problems present on the list, try rostopic echo <topic> or in your case rostopic echo /scan.
If nothing is published on the topic, you should then dorostopic info <topic> and check which node is responsible for the messages published to the topic.

Then, you should do rosnode info <node>, and set the node as the one responsible for publishing on the topic. Next, you should see if the node needs some data in order to publish to the problematic topic. What I want to say is, maybe the node publishing the /scan needs more information before it actually sends messages to the topic.

As a suggestion, you should always know exactly which node publishes which topics and how. Usually, problems like this are results of not really being sure why is something published and how exactly your nodes work.

Please check all the thing I've said and let me know if it helps.

If you have any further questions, feel free to ask and I will edit my answer.