Hi Tom,
One more thing to try. In case your systems are connected to multiple networks (i.e. have multiple IP addresses), could you try starting them with the gemfire property bind-address set to the IP address for the network which is common to all of the members. Using gfsh it would be something like:
start server --name=server1 --locators=localhost[19991] --J=-Dgemfire.bind-address=192.168.162.1
--Jens