Creating java FTP client; problem with firewall and PASV mode

[Jimmy Schmoe]

I've successfully created a nice little java FTP client (actual FTP
<RFC959>, not the URL version Java supports by default <RFC1738>). I
wrote it to support both PORT and PASV modes, and it works
successfully as long as no firewall is between the client and server.
The firewall blocks PORT communication, which is standard and
expected. For some reason, though, all firewalls this has been tested
with block the port assigned by the FTP server through the PASV
command, even though the same setup works fine from MS command line
FTP using PASV as well as other clients (tested with CuteFTP). Does
anyone have any ideas why the socket opened with Java would be blocked
while sockets opened with other programs aren't blocked? The firewall
is reporting that the expected port is being blocked, even though it
should be picking up the port to open when it sends the response to
the PASV command through -- the java client does get the PASV response
successfully, and seems to be attempting to open a socket on the
correct port, as shown by the firewall. Is there any reason this
should be happening, and what can I do to fix it?

Thanks for any help given,

Jimmy