Adding Headers and Post data in RESTfull/HTTP Request in C#

I'm having problems with sending POST request in C# and it seems I misunderstood some HTTP basics. So basically I'm implementing RESTfull service client, which work as follows:

1. Make POST request with username/password and get token
2. Use this token in header (Authorization:TOKEN) while making other GET/POST/PUT requests

I use WebRequest to make GET requests (with Authorization header) and it's working. But when I use following code to make PUT requests, service is giving "Authentication failed - not logged in" message back:

String url = String.Format("{0}/{1}", AN_SERVER, app);
WebRequest theRequest = WebRequest.Create(url);
theRequest.Method = "POST";

theRequest.ContentType = "text/x-json";
theRequest.ContentLength = json.Length;
Stream requestStream = theRequest.GetRequestStream();

requestStream.Write(Encoding.ASCII.GetBytes(json), 0, json.Length);
requestStream.Close();


theRequest.Headers.Add("Authorization", authToken);

HttpWebResponse response =  (HttpWebResponse)theRequest.GetResponse();

I must be making minor mistake (at least I hope so) while sending POST request. So what am I doing wrong?

Thanks.