You use nmap and get this error from one server but not another:
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 3.05 seconds
There seems to be no intermediate firewall rule stopping one server. What should you do?
The error message suggests using "-Pn". We recommend you try that:
nmap -Pn x.x.x.x
# (where x.x.x.x is the IP address of the server with the problem)
You may want to try "sudo " before your nmap command; the results may differ. It is possible that ICMP packets have been turned off in the intermediate networking devices.