Impossible to get a valid response from the third action of oauth connection

i have written a Vb.net code. the first two step are ok but the third always failed.

could somebody help us ?

the code of the step 3 (retrieve access token)
============================================================================
'Step 3:  Retrieve Access Token

        Console.WriteLine()
        Console.WriteLine("'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''")
        Console.WriteLine("Step 3:  Retrieve Access Token")

        normalized_url = ""
        normalized_request_params = ""
        oauth_timestamp = oauthHelper.GenerateTimeStamp()
        'oauth_timestamp = ""
        oauth_nonce = oauthHelper.GenerateNonce()

        signature = oauthHelper.GenerateSignature(New System.Uri(request_token_url), consumer_key, consumer_secret, _
                                                  oauth_token, oauth_token_secret, "POST", oauth_timestamp, oauth_nonce, _
                                                  OAuth.OAuthBase.SignatureTypes.PLAINTEXT, normalized_url, normalized_request_params)

        Console.WriteLine("signature = " & signature)
        Console.WriteLine("oauth_timestamp = " & oauth_timestamp)
        Console.WriteLine("oauth_nonce = " & oauth_nonce)
        Console.WriteLine("normalized_url=" & normalized_url)
        Console.WriteLine("normalized_request_params=" & normalized_request_params)

        request_params = "oauth_consumer_key=" & consumer_key & _
                         "&oauth_token=" & oauth_token & "&oauth_signature=" & signature & _
                         "&oauth_signature_method=PLAINTEXT" & "&oauth_timestamp=" & oauth_timestamp & _
                         "&oauth_nonce=" & oauth_nonce & "&oaut_version=1.0"

        Dim access_request As HttpWebRequest = HttpWebRequest.Create(access_token_url)

        access_request.Method = "POST"
        access_request.ContentType = "application/x-www-form-urlencoded; charset=utf-8"
        access_request.Headers.Add("oauth_signature", signature)
        access_request.Headers.Add("oauth_signature_method", "PLAINTEXT")
        access_request.Headers.Add("oauth_timestamp", oauth_timestamp)
        access_request.Headers.Add("oauth_nonce", oauth_nonce)
        access_request.Headers.Add("oauth_version", "1.0")

        writer = New StreamWriter(access_request.GetRequestStream())
        writer.Write(request_params)
        writer.Close()

        Try
            response = access_request.GetResponse()
        Catch ex As WebException
            response = ex.Response
        End Try

        Console.WriteLine("status code = " & response.StatusCode.ToString)
        Console.WriteLine("status desc = " & response.StatusDescription)
        stream = New StreamReader(response.GetResponseStream())
        data = stream.ReadToEnd()

        Console.WriteLine("data = " & data)

        response.Close()
===========================================================================


Log result

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
 Step 1:  Retrieve Request Token
signature = ZZEFLBfilhzigssgvssv%26
oauth_timestamp = 1358266410
oauth_nonce = 9180800
normalized_url=
normalized_request_params=
address = https://10.10.12.222:9443/jts/oauth-request-token?
uri = https://10.10.12.222:9443/jts/oauth-request-token?
status code = OK
status desc = OK
data = oauth_token=54cf94da29b8412283bb1238f8399c47&oauth_token_secret=180eAQvdZHWSDi4VlNl1c0nc1iKyuw2UGuKqfMsTqbE%3D
oauth_token        = 54cf94da29b8412283bb1238f8399c47
oauth_token_secret = 180eAQvdZHWSDi4VlNl1c0nc1iKyuw2UGuKqfMsTqbE%3D

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Step 2:  Authenticate Request Token
Go to the following URL to authenticate:
uri = https://10.10.12.222:9443/jts/oauth-authorize?oauth_token=54cf94da29b8412283bb1238f8399c47

Press enter to continue:
status code = OK
status desc = OK

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Step 3:  Retrieve Access Token
signature = ZZEFLBfilhzigssgvssv%26180eAQvdZHWSDi4VlNl1c0nc1iKyuw2UGuKqfMsTqbE%253D
oauth_timestamp = 1358266410
oauth_nonce = 8526939
normalized_url=
normalized_request_params=
status code = Unauthorized
status desc = Non-Autorisé
data = oauth_problem=invalid_expired_token