Can't load a page with httpwebrequest - vb.net

Question

I want to take the Flight schedule from the following address:
http://fo-apac.ttinteractive.com/Zenith/FrontOffice/(S(nves1yv4xxoia40cmotixof1))/USBangla/en-GB/BookingEngine/SearchFlights?__cnv=tShqK&json={"BookingPathArguments":null,"OriginDestinations":[{"IsOpen":false,"DataIdOrigin":6337,"DataIdDestination":6707,"DateTime":"2016-04-27T00:00:00.000"}],"TravelerTypes":[{"DataId":1,"TravelerCount":1},{"DataId":2,"TravelerCount":0},{"DataId":3,"TravelerCount":0}],"Currency":{"Code":"BDT"},"PromoCode":null,"DisplayRealAvailability":false,"Visibility":0,"ExtendedSearchDayCount":3}
If you paste the address to browser address bar and use firebug (or any fiddler), you will see this page sends 3 jquery ajax calls to bring the schedule. The following ajax POST actually fetch the schedule.
http://fo-apac.ttinteractive.com/Zenith/FrontOffice/(S(nves1yv4xxoia40cmotixof1))/USBangla/en-GB/FlexibleFlightStaticAjax/FlexibleFlightListLoadSelectedDays?__cnv=mxw0s
PostData : SaleConditionAccepted=false&ExtendedSearchDayCount=3&DoNotCheckCache=false&AlreadyLoggedIn=false&TempDataGuid=nves1yv4xxoia40cmotixof1&CurrencyCode=BDT&FareBasisDataId=&Travelers[0][DataId]=1&Travelers[0][Count]=1&UserSelections[0][SelectedDate]=2016-04-27T00:00:00&UserSelections[0][ReferenceDate]=2016-04-27T00:00:00&UserSelections[0][DataIdOrigin]=6337&UserSelections[0][DataIdDestination]=6707&UserSelections[0][GenericClassDataId]=&UserSelections[0][SelectedSegments]=&JsonPrepareBookingRequest=&PromoCode=
I am sending the request with httpwebrequest, but for unknown reason, I miss the session. I used CookieContainer to keep the cookies. I used the following function to send httprequest:
Public Function GetPostWP(ByVal Url As String, ByVal CkCont As CookieContainer, Optional ByVal PostData As String = "", Optional ByVal refSite As String = "") As String
Dim pStr As String = ""
Try
Dim Http As HttpWebRequest = WebRequest.Create(Url)
If refSite <> "" Then Http.Referer = refSite
Http.Headers.Add(HttpRequestHeader.AcceptEncoding, "gzip,deflate")
Http.CookieContainer = CkCont 'Initial CkCont is Nothing
Http.KeepAlive = True
Http.AllowAutoRedirect = True
'Http.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2) Gecko/20100115 Firefox/3.6"
Http.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
Http.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.137 Safari/537.36"
If PostData <> "" Then
Http.Method = "POST"
Http.ContentLength = PostData.Length
Http.ContentType = "application/x-www-form-urlencoded"
Dim PostStream As StreamWriter = New StreamWriter(Http.GetRequestStream())
PostStream.Write(PostData)
PostStream.Close()
End If
Using WebResponse As HttpWebResponse = Http.GetResponse()
Dim responseStream As Stream = WebResponse.GetResponseStream()
If (WebResponse.ContentEncoding.ToLower().Contains("gzip")) Then
responseStream = New GZipStream(responseStream, CompressionMode.Decompress)
ElseIf (WebResponse.ContentEncoding.ToLower().Contains("deflate")) Then
responseStream = New DeflateStream(responseStream, CompressionMode.Decompress)
End If
Dim reader As StreamReader = New StreamReader(responseStream, Encoding.Default)
pStr = reader.ReadToEnd()
responseStream.Close()
End Using
tmpCky = CkCont 'tmpCky is a Public CookieContainer Variable to hold cookies for future use.
GetPostWP = pStr
Catch ex As Exception
GetPostWP = "Error : " & ex.Message
End Try
End Function
Dim Cky As New CookieContainer
Dim Txt as String = GetPostWP(PostAddress, Cky, PostData, RefAdd)
Cky = tmpCky
Can anyone analyze the page, please?