No problem. There are several links to more info about PASV mode in this recent post:
The difference it can make has a lot to do with the success of the communication between the server and the client.
You might also want to take a good browse through the smartFTP forums and knowlegebase - lots of related stuff there. Just work off the earlier link I provided, and click "Support" in the left-side menu (I noticed "lots" of stuff related to possible firewall interaction, antivirus stuff, and other "problems" being discussed when I Googled it earlier).