IHttpAsyncHandler don't receive session cookies from asp web sites

I had make an reverse proxy using IHttpAsyncHandler class that listen on the iis request and according to the incoming url path i send a request to another site

public class ReverseProxy : IHttpAsyncHandler
{

    public bool IsReusable
    {
        get
        {
            return true;
        }
    }

    public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
    {
        _context = context;

       //some code
        remoteUrl = GetRemoteUrl(context, ref remoteUrl);

        _request = (HttpWebRequest)WebRequest.Create(remoteUrl);
        _helper.CopyHeadersRequest(context.Request, _request, ref  _newUri);
        // Copy POST Data
        if (context.Request.RequestType == "POST")
        {
            _request.ContentLength = context.Request.ContentLength;
            _request.ContentType = "application/x-www-form-urlencoded";
            _helper.CopyStream(context.Request.InputStream, _request.GetRequestStream());
        }

        return _request.BeginGetResponse(cb, extraData);
    }

    private string GetRemoteUrl(HttpContext context, ref string remoteUrl)
    {
       //get the new site url
    }

    public void EndProcessRequest(IAsyncResult result)
    {
        HttpWebResponse response;
        try
        {

            response = (HttpWebResponse)_request.EndGetResponse(result);

            _helper.CopyHeadersResponce(response, _context, ref  _segments);

         //some other code
        }
        catch (System.Net.WebException we)
        {
            Return404(_context);
            return;
        }
    }

the problem is when i call an remote asp website the response come back without the session cookies (ASP.NET_SessionId). and this remote website lose the session data. What can be the problem ?