It’s common for a Windows-to-IBM i FTP transfer to get blocked when the Windows desktop firewall blocks port 20 traffic to the Windows desktop (port 20 is used in the FTP data transfer).
You may even see your FTP session connect to your IBM i partition but fail when it tries to transfer data. Sometimes the desktop will display an FTP 425 error–Unable to open data connection. This recently happened to a client of mine who was having trouble performing an FTP upload from a FedEx server on their warehouse floor.
The best way to take care of this is to turn off the Windows firewall on the blocked machine. That should allow you to complete the transfer.
(originally posted on May 1, 2013. Reposted on November 18, 2013)