net_adm:ping not working

[erlang]

I can't get the ping-pong distributed example to work, so I tried simply 
pinging another node. This doesn't work either.

os: windows server 2003, machine name is my-server.
EShell v5.5.1

command lines: erl -sname a
                            erl -sname b

(b@REDACTED)1> net_adm:ping('a:my-server').

always returns pang. The cookie file is being read, and the cookie is 
the same in both nodes. What to try next?