Why I am unable to connect to SFTP using Rebex in Windows server?

        SftpConnectionState IsActive = null;
        IsActive = uSftp.GetConnectionState();
        //End
        WriteLog("SFTP_Download() : Connecting Sftp:" + DateTime.Now.ToString("dd-MM-yyyy HH:mm:ss:fff"));

        string[] sSFTPHost = ConfigurationManager.AppSettings["SFTPHost"].Split('|');
        if (copyAction == "U")
        {

            //Added  on 18.8.2015 for Rebex
             if (IsActive.Connected == false)
             {
                 uSftp.Connect(sSFTPHost[0], Convert.ToInt32(sSFTPHost[1]));
                 uSftp.Login(sSFTPHost[2], sSFTPHost[3]);

            }
            //End
        }
        else
        {
            //Added  on 18.8.2015 for Rebex
            if (IsActive.Connected == false)
            {
                uSftp.Connect(sSFTPHost[0], Convert.ToInt32(sSFTPHost[1]));
                uSftp.Login(sSFTPHost[2], sSFTPHost[3]);

            }
            //End
        }
    }
    catch (Exception ex)
    {
        WriteLog("SFTP_Download() :Exception while connecting sftp:Exception:"+ex.Message.ToString() + DateTime.Now.ToString("dd-MM-yyyy HH:mm:ss:fff"));
        throw new System.Exception("ConnectSftp(): " + ex.Message);
    }
}