This is a obvious one but are you sure you have the right nodes in the correct group? Solarwinds will not show something as up unless it responds to a ping. Double check the IP address for the nodes in Portsmouth group and confirm that they are in fact located in Portsmouth. Do a traceroute and compare results to that one node that did show as being down.
Yes traceroute and extended ping should provide some clarity concerning the network connectivity. (Interesting trouble)
Never be concerned about asking the most obvious questions (kinda like me stating the obvious here huh?)
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