If you try to download a file from a folder shared to you by another user, please make sure that folder is still shared to you, and that the folder was shared with Download or a higher level permission.
If you try to download a file from your own account, there should be no permission issue at all. Please verify that you can download files using FileZilla first and check if your code is sending the same (or similar) FTP commands. We usually don't support your own code or custom software. But if you have a business level subscription, you can try to send an email to DriveHQ customer support.