FTPS SSL/TLS Issue on Some Computers

Hi
I am using this code to upload a file to my server but on some computers like 1% of users it doesn't upload the file to the server. After trying to connect on that PC which has the problem, with Filezilla to my server I see that it fails to make the connection because it can't pass the SSL/TLS authentication part.

public static void Upload(string strLocalFileName,string strRemoteFileName, string strUsername, string strPassword)
{
WebRequest wreq = WebRequest.Create(HTTPloc.fileserverurl + strRemoteFileName);
FtpWebRequest ftp = (FtpWebRequest)wreq;

// This returns true always because my certificate is not signed, it is self-generated
ServicePointManager.ServerCertificateValidationCallback = RemoteCertificateValidationCallback;

ftp.Credentials = new NetworkCredential(strUsername, strPassword);

ftp.Method = WebRequestMethods.Ftp.UploadFile;

ftp.UsePassive = true;
ftp.UseBinary = true;
ftp.KeepAlive = false;
ftp.EnableSsl = true;

FileStream stream = File.OpenRead(strLocalFileName);
byte[] buffer = new byte[stream.Length];

stream.Read(buffer, 0, buffer.Length);
stream.Close();

Stream reqStream = ftp.GetRequestStream();
reqStream.Write(buffer, 0, buffer.Length);
reqStream.Close();
}

The exception it throws is something like "remote computer didn't answer in the time provided" actually it defines timeout.

User tried OS reinstall but still the same problem. It has to be a problem with the things he does after installation. Any ideas? What causes a problem on SSL TLS authentication?

Thanks.