If you are running multiple nodes on the same external IP address you might have problems connecting. Such a situation can happen if you are running multiple nodes in the same apartment, office, or a similar local network which presents one external IP address.
A workaround for this is to change the default port the node listens on, or alternatively change the external port that the node advertises. These can be changed with --listen-port and --external-port flags, respectively. The default value for --listen-port is 8888 and the default value for --external-port is the value of --listen-port.