NAT routers don't play well with site to site transfers.
There are a couple of ways around this problem, however they all require changes to the ftp server.
1. run the ftp server on a port other than 21. usually NAT only monitors port 21
2. connect to the site using ssl/tls, since the connection is encrypted NAT wont be able to do anything.
Basically what happens is that NAT is designed to prevent the ftp client from sending the wrong info to the ftp server. When the client sends the PORT command the NAT router modifies the packet to include the external IP of the router. This is fine for normal transfers but site to site transfers require the PORT command to be left as-is, since the IP address points to a ftp server.
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