Lost session in webservice

[dockuvn]

I use a webservice project and add refference it to presentation project.
In webservice, I use session to store a value. Howerver, it cannot be keeped.
Please open my sollution and help me ASAP.
Thanks
Webservice Code

Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.ComponentModel

<System.Web.Services.WebService(Namespace:="http://tempuri.org/")> _
<System.Web.Services.WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
<ToolboxItem(False)> _
Public Class Service1
    Inherits System.Web.Services.WebService

    <WebMethod(True)> _
    Public Function HelloWorld() As String
        Setsession("Thank You")
        Return ""
    End Function
    <WebMethod(True)> _
    Public Function getses() As String
        Return GetSession()
    End Function
    Private Sub Setsession(ByVal value As String)
        Session.Add("a", value)
    End Sub
    Private Function GetSession() As String
        Return Session("a").ToString
    End Function
End Class

Presentation Code

Partial Class _Default
    Inherits System.Web.UI.Page

    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim a As New localhost.Service1
        Dim s As String = a.HelloWorld
        Response.Write(a.getses())
    End Sub

    Protected Sub Button2_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim a As New localhost.Service1
        Response.Write(a.getses())
    End Sub
End Class