The first and most obvious thing to check is that there are actually files in your account. If this is not the problem, you should also check that you have set a root folder for the account. However, the most common cause of this problem is a router or firewall. FTP connections use two individual connections; one for commands and the other for data (such as files and directory listings.) If the data connection is blocked by a router or firewall, then you will not be able to view the contents of any folder. Please read
How do I set up my router / firewall for Ability FTP Server? for more details.