Web Forms :: GridView Sort Is Working Only First Time

My Gridview sort  is working only first time. My Gridview sort only first time when I click on any column

I want ASC > DESC 

Again DESC > ASC 

and Again ASC > DESC this way my sort should work 

I have changed my code 

I added Gridview binding to PAge_Load function 

Code is below 

Public Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim myUserInfo As GCS_User_Info = Session("UI")
        If Not Page.IsPostBack Then
               GRDView.DataSource = GetSortableData(String.Empty) 'System.Drawing.FontFamily.Families
               GRDView.DataBind()

[Code] ....