How Do You Troubleshoot the nmap Results “Host seems down” when the Other Server is Not Down?

Problem scenario
Two servers have a specific incoming port open between them. But when you try nmap to test the port, you get a message like this:

Starting Nmap 7.60 ( ) at 2019-10-08 19:57 UTC
Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn
Nmap done: 1 IP address (0 hosts up) scanned in 3.04 seconds

What should you do?

Possible solution #1
Open more ports between the two hosts. If a firewall or security group is blocking some ports, this could happen. Surgically opening the port you are testing in an Amazon Security Group would not be sufficient to have nmap show it is open.

Possible solution #2
Try internal or external IP addresses.

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