Put a debug node on the mqtt In node and anther on the Change node. Give them names so they are easier to identify in the debug pane. Then look at what you are getting out of each and make sure it is what you expect. If that looks good then add another In node subscribing the the the topic you have configured for the Out node and check the right data is coming back (with another debug node). If you still can't work out what the problem is then post your flow here and explain what is not happening correctly. See this post for how to share a flow here.
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