Why doesn't the gridview persist the checkbox values between pagination?

I am trying to store the checboxes values between paginations so if i go from 1 page to another and ocme back the the checkbox checked would persist.

Protected Sub gv_InforWO_PageIndexChanging(sender As Object, e As GridViewPageEventArgs)
       Try
            gv_InforWO.PageIndex = e.NewPageIndex

            If chkFiltersAvailable.Checked = False Then
                chkSelectAll.Checked = False
                BindGridInfoWO()
                'btnSync_Click(Nothing, Nothing)
            Else

                BindGridInfoWO()
                ' btnSync_Click(Nothing, Nothing)
            End If

            'checkbox persistence code
            Dim d As Integer = gv_InforWO.PageCount
            Dim values() As Boolean = New Boolean((gv_InforWO.PageSize) - 1) {}
            Dim chb As CheckBox
            Dim count As Integer = 0
            Dim i As Integer = 0
            Do While (i < gv_InforWO.Rows.Count)
                chb = CType(gv_InforWO.Rows(i).FindControl("chkSelect"), CheckBox)
                If (Not (chb) Is Nothing) Then
                    values(i) = chb.Checked
                End If

                i = (i + 1)
            Loop

            Session(("page" + CType(gv_InforWO.PageIndex, String))) = values


        Catch ex As Exception
             lblWaitingMsg.Text= ex.Message 
        End Try
    End Sub

On pre-render

Protected Sub gv_InforWO_PreRender(sender As Object, e As EventArgs)
        Try
            If (Not (Session(("page" + CType(gv_InforWO.PageIndex, String)))) Is Nothing) Then
                Dim chb As CheckBox
                Dim values() As Boolean = CType(Session(("page" + CType(gv_InforWO.PageIndex, String))), Boolean())
                Dim i As Integer = 0
                Do While (i < gv_InforWO.Rows.Count)
                    chb = CType(gv_InforWO.Rows(i).FindControl("chkSelect"), CheckBox)
                    chb.Checked = values(i)
                    i = (i + 1)
                Loop

            End If
        Catch ex As Exception

        End Try
       
    End Sub

but it doesn't persist the value and comes as false even if I tick the boxes.