Allow paging of a GridView does not move to the record on the next page.

This is my Gridview:
      <asp:GridView ID="GrdVwBuyUpContributions" runat="server" AutoGenerateColumns="False"
                                                           
            OnRowEditing="GrdVwBuyUpContributions_Click" OnRowUpdating="GrdVwBuyUpContributions_update" OnRowCancelingEdit="GrdVwBuyUpContributions_RowCancelingEdit" OnPageIndexChanging="GrdVwBuyUpContributions_PageIndexChanging"
            BackColor="White" BorderColor="#E7E7FF"
            BorderStyle="None" BorderWidth="1px" GridLines="Horizontal" HorizontalAlign="Left" DataKeyNames="Contribution_ID"
            Width="300" AllowPaging="True" PageSize="10" >
            <Columns>
                <asp:CommandField ShowEditButton="True"></asp:CommandField>
                <asp:TemplateField HeaderText="Contribution Date" HeaderStyle-HorizontalAlign="Left">
                    <ItemTemplate>
                        <%# Eval("Date") %>
                    </ItemTemplate>
                    <EditItemTemplate>
                        <asp:TextBox ID="DateTextBox" Text='<%# Eval("Date")%>' runat="server"></asp:TextBox>
                        <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="You must enter a valid date."
                            ControlToValidate="DateTextBox">
                            *</asp:RequiredFieldValidator>
                        <asp:CompareValidator ID="CompareValidator1" runat="server" ErrorMessage="You must provide a valid Date.."
                            ControlToValidate="DateTextBox" Operator="DataTypeCheck" Type="Date">
                            *</asp:CompareValidator>
                        <cc1:CalendarExtender ID="CalendarExtender1" TargetControlID="DateTextBox" Format="yyyy/MM/dd"
                            runat="server">
                        </cc1:CalendarExtender>
                     
                    </EditItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Amount" HeaderStyle-HorizontalAlign="Left">
                    <ItemTemplate>
                       <%# string.Format("{0:c}", Eval("Amount"))%>
                    </ItemTemplate>
                    <EditItemTemplate>
                        <asp:TextBox ID="AmountTextBox" Text='<%# string.Format("{0:0.00}", Eval("Amount"))%>' runat="server"></asp:TextBox>
                        <asp:RequiredFieldValidator ID="RequiredFieldValidator2"  runat="server" ErrorMessage="You must enter an amount."
                            ControlToValidate="AmountTextBox">
                            *</asp:RequiredFieldValidator>
                        <asp:CompareValidator ID="CompareValidator2" runat="server" ErrorMessage="You must provide a valid currency value for the Amount."
                            ControlToValidate="AmountTextBox" Operator="DataTypeCheck" Type="Currency">
                            *</asp:CompareValidator>
                    </EditItemTemplate>
                </asp:TemplateField>
            </Columns>
            <FooterStyle BackColor="#B5C7DE" ForeColor="#4A3C8C" />
            <RowStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" />
            <PagerStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" HorizontalAlign="Right" />
            <SelectedRowStyle BackColor="#738A9C" Font-Bold="True" ForeColor="#F7F7F7" />
            <HeaderStyle BackColor="#4A3C8C" Font-Bold="True" ForeColor="#F7F7F7" />
            <AlternatingRowStyle BackColor="#F7F7F7" />
        </asp:GridView>

This is the code when pageIndex changes:

 protected void GrdVwBuyUpContributions_PageIndexChanging(Object sender, GridViewPageEventArgs e)
     {

       // Cancel the paging operation if the user attempts to navigate
       // to another page while the GridView control is in edit mode.
       if (GrdVwBuyUpContributions.EditIndex != -1)
       {
         // Use the Cancel property to cancel the paging operation.
         e.Cancel = true;

         // Display an error message.
         int newPageNumber = e.NewPageIndex + 1;
         lblMessage.Text = "Please update the record before moving to page " +
           newPageNumber.ToString() + ".";
       }
       else
       {
         // Clear the error message.
         lblMessage.Text = "";
         
       }

However when I click on any of the page numbers on the page, the gridview does not do what it is supposed to do.

Why?