Why Gridview Page Index Chaingin doesn't work !!

<%@ Page Title="Search Business Type" Language="C#" MasterPageFile="~/ydgMasterPage.master"
    AutoEventWireup="true" CodeFile="SearchBusinessType.aspx.cs" Inherits="SearchBusinessType" %>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    <link href="css/gridView.css" rel="stylesheet" type="text/css" />
    <div align="center">
        <asp:Label ID="lblSearching" runat="server" ForeColor="Red" Font-Size="Large">Search By Business Type</asp:Label><br />
        <asp:LinkButton ID="lnkBtnA" runat="server" OnClick="lnkBtnA_Click">A</asp:LinkButton>
        <asp:LinkButton ID="lnkBtnB" runat="server" OnClick="lnkBtnB_Click">B</asp:LinkButton>
        <asp:LinkButton ID="lnkBtnC" runat="server" OnClick="lnkBtnC_Click">C</asp:LinkButton>
 </div>
    <br />
<div align="center">
        <asp:Label ID="Message" ForeColor="Red" runat="server" />
        <asp:GridView ID="gridAlphaBetical" runat="server" EmptyDataText="There is no Data."
            AutoGenerateColumns="False" Width="500px"AllowPaging="true"
            EnableModelValidation="True" OnPageIndexChanging="gridAlphaBetical_PageIndexChanging1">
            <Columns>
                <asp:BoundField DataField="New Business Type" HeaderText="Business Type(Eng)" />
                <asp:TemplateField>
                    <ItemTemplate>
                        <asp:LinkButton ID="lnkView" runat="server" CommandArgument="<%#Bind('Sr') %>" OnCommand="lnkView_Click"
                            Text="View" ViewStateMode="Enabled"></asp:LinkButton>
                    </ItemTemplate>
                </asp:TemplateField>
            </Columns>
        </asp:GridView>
    </div>
</asp:Content>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Web.Security;
using System.Data.SqlClient;
using System.Web.Configuration;

public partial class SearchBusinessType : System.Web.UI.Page
{
    protected void gridAlphaBetical_PageIndexChanging1(object sender, GridViewPageEventArgs e)
    {
        if(Page.IsPostBack)
        {
            gridAlphaBetical.PageIndex = e.NewPageIndex;

        }

    }

 public void BindGridData_A()
    {

            string constring = WebConfigurationManager.ConnectionStrings["ydConnectionString"].ToString();
            SqlConnection connection = new SqlConnection(constring);

            SqlCommand cmd = new SqlCommand();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.SelectCommand = cmd;
            cmd.Connection = connection;
            DataTable dtA = new DataTable();

            cmd.CommandText = "SELECT * FROM BusinessCode WHERE [New Business Type] LIKE 'A%'";

            da.Fill(dtA);
            gridAlphaBetical.DataSource = dtA;
            gridAlphaBetical.DataBind();

    }
    public void BindGridData_B()
    {

            string constring = WebConfigurationManager.ConnectionStrings["ydConnectionString"].ToString();
            SqlConnection connection = new SqlConnection(constring);

            SqlCommand cmd = new SqlCommand();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.SelectCommand = cmd;
            cmd.Connection = connection;
            DataTable dtB = new DataTable();


            cmd.CommandText = "SELECT * FROM  BusinessCode WHERE [New Business Type] LIKE 'B%'";

            da.Fill(dtB);
            gridAlphaBetical.DataSource = dtB;
            gridAlphaBetical.DataBind();

    }
    public void BindGridData_C()
    {

            string constring = WebConfigurationManager.ConnectionStrings["ydConnectionString"].ToString();
            SqlConnection connection = new SqlConnection(constring);

            SqlCommand cmd = new SqlCommand();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.SelectCommand = cmd;
            cmd.Connection = connection;
            DataTable dtC = new DataTable();


            cmd.CommandText = "SELECT * FROM  BusinessCode WHERE [New Business Type] LIKE 'C%'";

            da.Fill(dtC);
            gridAlphaBetical.DataSource = dtC;
            gridAlphaBetical.DataBind();

    }

 protected void lnkBtnA_Click(object sender, EventArgs e)
    {

          BindGridData_A();


    }
    protected void lnkBtnB_Click(object sender, EventArgs e)
    {


            BindGridData_B();

    }
    protected void lnkBtnC_Click(object sender, EventArgs e)
    {
            BindGridData_C();
    }