There are a couple of things I would try:
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Try increasing the packet size on the underlying pubsubclient: Sending commands with larger payloads Unfortunately the underlying library silently refuses commands if the packet is too large.
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Does the command show up in the application communication log? This log is a live stream of all device communication. Open it another tab and then attempt to send the command. It should show up.
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Try manually sending the command using the debug tab on the device page.