Refresh token is missing or invalid


I am getting refresh token is missing or invalid error message using API calls always.
Here is the code. Can you please help me to solve this issue.

        string BaseUrl = "";      
        string   RefreshToken = "1:2:1:2:qBdm2W1+YKY3TuVHKBvVmd82ZMw7GDOAXiWmYqfpOlB1EHEl1uovEm/6gwNpqhlZ0fInqO5sQzd0sd+6atjYSu6LfJRVHyfCAjQ48M6OEgo="; 
        HttpWebRequest request = ( HttpWebRequest ) WebRequest.Create(BaseUrl);
        request.Method = "POST";
        request.ContentType = "application/x-www-form-urlencoded; charset=UTF-8"; 
        string PostData = "grantType=REFRESH_TOKEN&refreshToken=" + RefreshToken;
        byte[] buf = Encoding.UTF8.GetBytes(PostData);
        request.GetRequestStream().Write(buf, 0, buf.Length);
        var response = ( HttpWebResponse ) request.GetResponse();
        var responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();

What is the use of Poynt-Request_Id by passing in header?Is there any expire time for Poynt-Request_Id ?

FYI: Sometimes working and sometimes not working the POST actions. We are facing lot of issues with Poynt integration from the last 20 days.