"Unable to connect to the remote server" when using FtpWebRequest

I am writing an application to generate a XML file from database then ftp it onto a FTP server. I encounter problem during the deployment.

The code fragments for the FTP is:
// Create FtpWebRequest object from the Uri provided
reqFTP = (FtpWebRequest)FtpWebRequest.Create(new Uri("ftp://" + ftpServerIP +
"/" + fileInf.Name));
// Provide the WebPermission Credintials
reqFTP.Credentials = new NetworkCredential(ftpUserID, ftpPassword); 
1. I deploy the project onto the live web server which is hosted by 3rd party. I point the FTP server to the same server(we call it server A). It works perfect.
2. But when I point the FTP server to a different one (we call it server B), it returns an error message "Unable to connect to the remote ser ...