I can't write to a server which is hosted off an ADSL modem. I can connect and copy files FROM the server but can't write TO it.

My modem has a virtual server setting which points FTP to my FTP Server on port 21. This seems to work fine. I am assuming there is some return communication which is failing.

I have read a few articles about passive and active connections but i don't get it? I am trying the software, if I get it to work I will purchase. Any ideas on how I can fix this issue? it is probably a simple fix.