Are you adding the new node by IP or hostname? If via a hostname, check that all nodes in the cluster resolve that hostname to the same IP address.
Your thought on firewall is also a possibility - there are additional ports used by 2.2.0 compared to 2.0. See http://docs.couchbase.com/couchbase-manual-2.2/#network-ports for a list of ports which need to be open. I’d suggest dropping the firewall to test if that solves the problem.