odd behavior

I have publicfile configured on a machine behind a firewall/router
as an ftp server.  I've configured the firewall to direct ftp
traffic to the machine with publicfile running.

From the internal network, I can connect to publicfile, go through
the faux authentication, cd, ls, and get.

From external networks, I can connect to publicfile and go through
the authentication, change directories, but pretty much nothing else.

For example:

ftp triceratops.com
Connected to triceratops.com.
220 Features: a p .
Name (triceratops.com:nobody): 
230 Hi. No need to log in; I'm an anonymous ftp server.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
200 Okay.
150 Making transfer connection...

And it hangs there.

Now, my guess is that this is a firewall issue, but I -am- able to
connect, which makes me doubt that this is the problem.  Is there
another component to the ftp connection that's getting filtered
somehow?  Any thoughts?

--
John White