Hi blondeastheycome, welcome to Firewall.cx.
Passive FTP differs from 'active' FTP in that the data channel can use any port by negotiation as opposed to port 20 which is the normal FTP data channel. Its possible that your application is trying to use a port that the router is blocking hence the failure.
Have a read at
slacksite.com/other/ftp.html
for a really good explanation.
To prove it conclusively try changing the port if your FTP application supports that. You could also try using the debug commands on the router to watch what's going on, or do a packet capture using something like Ethereal
