PageIndexChanging not working in Gridview

Question

I have gridview 

 <asp:GridView ID="gvArticles" runat="server" DataKeyNames="ArticleID" 

        HeaderStyle-BackColor="#87cfe6" AutoGenerateColumns="false" ForeColor="White" 

        AllowPaging="true" AllowSorting="true" PageSize="3" Width="100%">

        <Columns>

    <asp:TemplateField>

    <ItemTemplate>

        <asp:ImageButton ID="imgDel" runat="server" CommandName="Delete" ImageUrl="~/Images/delete.jpg" Width="20px" Height="20px"/>

    </ItemTemplate>

    </asp:TemplateField>

    <asp:TemplateField HeaderText="Title">

    <ItemTemplate>

        <asp:Label ID="lblTitle" runat="server" Text='<%#Eval("Title")%>'></asp:Label>

    </ItemTemplate>

    </asp:TemplateField>

    <asp:TemplateField HeaderText="Added Date">

    <ItemTemplate>

        <asp:Label ID="lblDate" runat="server" Text='<%#Eval("AddedDate") %>'></asp:Label>

    </ItemTemplate>

    </asp:TemplateField>

    </Columns>

    </asp:GridView>

and this is the code for .cs file 

public partial class DeleteArticles : System.Web.UI.Page

{

    string str = ConfigurationManager.ConnectionStrings["connect"].ConnectionString;

    SqlConnection con;

    SqlCommand cmd;

    protected void Page_Load(object sender, EventArgs e)

    {

        if (!IsPostBack)

        {

            gvArt();

        }

    }

    protected void gvArt()

    {

        con = new SqlConnection(str);

        con.Open();

        cmd = new SqlCommand("select * from Articles",con);

        SqlDataAdapter da = new SqlDataAdapter(cmd);

        DataSet ds = new DataSet();

        da.Fill(ds);

        gvArticles.DataSource = ds;

        gvArticles.DataBind();

        con.Close();

    }

    protected void gvArticles_PageIndexChanging(object sender, GridViewPageEventArgs e)

    {

       

    }

}

but when i try to change pageindex it doesn't change .