First, please consider using an uploading method other than FTP, such as SFTP or SCP. FTP is a complex, insecure protocol that does not work very well on the modern Internet. If you have an alternative, use it.
Please also keep in mind that, as of May 2010, memberships now come with FTP disabled by default, so if you must use it, you'll need to make sure it's properly enabled first.
With that said, almost all FTP problems are caused by NAT devices (especially cheap residential gateway routers) incorrectly processing FTP connections, or by personal firewall software blocking FTP access. These problems generally manifest as being able to connect to the FTP server, but not to upload or download any files. FTP has two operating modes, active and passive. The first response to any FTP connection problem should be to switch your FTP client from active mode to passive mode, or vice versa. Switching modes almost always clears up FTP issues.
Other potential problem areas for FTP include your local firewall configuration (hardware or software) and incorrect FTP connection information.