If you cannot upload to an FTP server:
Click on the Test FTP Settings button from Folder Properties dialog.
Check all your server details thoroughly, including the directory.
Check your server uses passive FTP.
Also, you may want to know that by default Fling uses the file's modification time and the archive flag to determine if upload is required. Most problems occur if a program or copy does not set the modified or archive flag. If this happens, right-click on the file and select Fling->Upload.