Paging in Gridview not working..

[kieky]

This is my code :

.aspx

<asp:GridView ID="GridView_Main" ClientIDMode="Static" AutoGenerateColumns="false"
                        DataKeyNames="IdVersi" AllowPaging="true" AllowSorting="true" Width="97%" BorderWidth="1"
                        OnSelectedIndexChanged="GridView_Main_SelectedIndexChanged" OnSorted="GridView_Main_Sorted"
                        OnPageIndexChanged="GridView_Main_PageIndexChanged" OnPageIndexChanging="GridView_Main_PageIndexChanging"
                        runat="server" PageSize="4">
                        <EmptyDataTemplate>
                            <div class="EmptyDataMessage">
                                <asp:Label ID="Label_GridView_Empty" runat="server" Text="Data tidak ditemukan."></asp:Label>
                            </div>
                        </EmptyDataTemplate>
                        <HeaderStyle CssClass="GridviewHeader" />
                        <SelectedRowStyle CssClass="GridviewSelect" />
                        <AlternatingRowStyle CssClass="GridviewAlternate" />
                        <Columns>
                            <asp:TemplateField HeaderText="No Versi" ItemStyle-CssClass="GridviewItem" ItemStyle-VerticalAlign="Top"
                                ItemStyle-HorizontalAlign="Center" HeaderStyle-Width="15%" ItemStyle-Wrap="false">
                                <ItemTemplate>
                                    <asp:LinkButton ID="LinkButton_Select" CommandName="Select" runat="server"><%# Eval ("NoVersi") %></asp:LinkButton>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:BoundField DataField="Notes" HeaderText="Catatan" ItemStyle-CssClass="GridviewItem"
                                HeaderStyle-Width="30%" ItemStyle-VerticalAlign="Top" ItemStyle-HorizontalAlign="Left" />
                            <asp:BoundField DataField="ModifiedOn" HeaderText="Diubah tanggal" ItemStyle-CssClass="GridviewItem"
                                HeaderStyle-Width="30%" ItemStyle-VerticalAlign="Top" ItemStyle-HorizontalAlign="Left" />
                            <asp:BoundField DataField="Type" HeaderText="Type" ItemStyle-CssClass="GridviewItem"
                                HeaderStyle-Width="30%" ItemStyle-VerticalAlign="Top" ItemStyle-HorizontalAlign="Left" />
                            <asp:BoundField DataField="Size" HeaderText="Size" ItemStyle-CssClass="GridviewItem"
                                HeaderStyle-Width="25%" ItemStyle-VerticalAlign="Top" ItemStyle-HorizontalAlign="Left" />
                            <asp:TemplateField HeaderText="Aksi" ItemStyle-CssClass="GridviewItem" ItemStyle-VerticalAlign="Top"
                                ItemStyle-HorizontalAlign="Center" HeaderStyle-Width="20%" ItemStyle-Wrap="false">
                                <ItemTemplate>
                                    <a href='<%# Eval ("UrlFile") %>'>Download</a>
                                </ItemTemplate>
                            </asp:TemplateField>
                        </Columns>
                        <PagerSettings Mode="NumericFirstLast" FirstPageImageUrl="~/App_Themes/DMS/icon/icon_nav_first_on.gif"
                            FirstPageText="Pertama" LastPageImageUrl="~/App_Themes/DMS/icon/icon_nav_last_on.gif"
                            LastPageText="Akhir" NextPageImageUrl="~/App_Themes/DMS/icon/icon_nav_next_on.gif"
                            PreviousPageImageUrl="~/App_Themes/DMS/icon/icon_nav_prev_on.gif" />
                        <PagerStyle HorizontalAlign="Center" VerticalAlign="Middle" ForeColor="#80A0CF" />
                    </asp:GridView>

and .cs

protected void GridView_Main_DataBind(int NewPageIndex)
    {
        id_document = Convert.ToInt32(Request.QueryString[0]);
        DMSDataAccess db = new DMSDataAccess();
        var versi = db.DMSDataContext.Versis.Select(p => p).Where(p => p.IdDokumen == id_document);

        GridView_Main.PageSize = itemPerPage;
        GridView_Main.PageIndex = NewPageIndex;
        GridView_Main.DataSource = versi;
        GridView_Main.DataBind();
    }
    protected void GridView_Main_SelectedIndexChanged(object sender, EventArgs e)
    {
        Button_Update.Visible = false;
        Button_UpdateKepemilikan.Visible = false;
        Label_Modal_Title.Text = "Properties";
        MultiView_Content.SetActiveView(View_Versi);
        try
        {
            id_document = Convert.ToInt32(Request.QueryString[0]);
            string id = GridView_Main.SelectedValue.ToString();
            DMSDataAccess db = new DMSDataAccess();
            var doc = db.DMSDataContext.Dokumens.Select(p => p).Where(p => p.IdDokumen.Equals(id_document)).FirstOrDefault();
            var versi = db.DMSDataContext.Versis.Select(p => p).Where(p => p.IdVersi.Equals(id)).FirstOrDefault();

            string id_category = Convert.ToString(doc.IdCategory);
            Label_Value_VersiJudul.Text = doc.JudulDokumen;
            Label_Value_VersiType.Text = versi.Type;
            Label_Value_VersiSize.Text = Convert.ToString(versi.Size) + " Kb";
            Label_Value_VersiVersi.Text = Convert.ToString(versi.NoVersi);
            Label_Value_VersiAuthor.Text = GetNamaDosen(versi.UserName);
            Label_Value_VersiTanggal.Text = Convert.ToString(versi.ModifiedOn);
            Label_Value_VersiCategory.Text = GetNamaCategory(id_category);
            Label_Value_VersiDeskripsi.Text = doc.Deskripsi;
            if (versi.Notes != null)
            {
                Label_Value_VersiNotes.Text = versi.Notes;
            }
            else { Label_Value_VersiNotes.Text = "-"; }
        }
        catch { }
        UpdatePanel_Panel_Modal.Update();
        this.MPE.Show();
    }
    protected void GridView_Main_Sorted(object sender, EventArgs e)
    {
        GridView_Main_DataBind(GridView_Main.PageIndex);
        GridView_Main.SelectedIndex = -1;
    }
    protected void GridView_Main_PageIndexChanged(object sender, EventArgs e)
    {
        GridView_Main_DataBind(GridView_Main.PageIndex);
        GridView_Main.SelectedIndex = -1;
    }
    protected void GridView_Main_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        GridView_Main.PageIndex=e.NewPageIndex;
    }

what's wrong? can u help me?