FtpWebRequest won't change directory

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C#

FTP

hi I need to connect to a device via ftp, change directory to system and then get some files from it. I use code below

reqFTP = (FtpWebRequest)WebRequest.Create
    (new Uri("ftp://" + ftpServerIP + "/system/" + fileName));
reqFTP.Method = WebRequestMethods.Ftp.DownloadFile;
reqFTP.UseBinary = true;
reqFTP.UsePassive = false;
reqFTP.Credentials = new NetworkCredential(ftpUserID, ftpPassword);
FtpWebResponse response = (FtpWebResponse)reqFTP.GetResponse();

I also tried the Uri like this

Uri("ftp://" + ftpServerIP + "/%2fsystem/" + fileName)

but the device answers:

550 Cannot find the file /system/filename.\r\n

:( but when i use the socket programming and send the command

"CWD " + directoryName

I am able to get the file. what's wrong with FtpWebRequest??