Windows firewall using wrong IP for FTP

Hi,
We have MS FTP on a dedicated IP that is not the NICs default/master IP.
Connecting to the IP is fine with Win firewall on, however the responding IP
is the master/default IP and not the other IP that is set for MS FTP.
This causes issues for end users (their firewalls etc) as they are receiving
a response from a different IP to the one they connected to.

Does all traffic coming from Windows firewall have to come from the master
IP? Why wont it respond from the IP the external connection was originally
made to?

Any help appreciated.