imagebutton on command doesnot fire / call function

<asp:TemplateField HeaderText="Action" >
                   <HeaderStyle width="10%" HorizontalAlign="center" CssClass="listheader" />
			          <itemstyle horizontalalign="center" />
	        	       <itemtemplate>
			          <asp:ImageButton id="deleter" ImageUrl="../images/icon_delete.gif" 
                      CausesValidation="False" runat="server"
				      AlternateText="Delete" ImageAlign="AbsMiddle" BorderStyle="None" Width="19" Height="19"
				      CommandArgument='<%# DataBinder.Eval(Container.DataItem, "c_slno") %>' 
                      OnCommand="delete_update"  CommandName="delete" />
		           </itemtemplate>

Sub delete_update(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.CommandEventArgs)
       
blah blah......

end sub