VS 2005 Imagebutton In Gridview Header Onclick Or Oncommand Event Not Firing

I have a gridview which is bound to a dataset. I have placed several template fields in the grid with the following type of setup

HTML Code:
<asp:TemplateField>
<HeaderTemplate>
<asp:Label ID="Label1" runat="server" Text="File"></asp:Label>
<asp:ImageButton AlternateText="ASC" ID="imgASC" Width="10" Height="10"
ImageUrl="~/Modules/Interface/ASC.gif"
CommandArgument="eFolder ASC" runat="server" OnCommand="ImageButton_Command" />
<asp:ImageButton AlternateText="DESC" ID="imgDESC" Width="10" Height="10"
ImageUrl="~/Modules/Interface/DESC.gif"
CommandArgument="eFile DESC" runat="server" OnCommand="ImageButton_Command" />
</HeaderTemplate>
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Eval("eFileName") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>

on the code page I have mapped the OnCommand event to the following code

Code:

Protected Sub ImageButton_Command(ByVal sender As Object, _
ByVal e As System.Web.UI.WebControls.CommandEventArgs)
Try
Dim img As ImageButton = CType(sender, ImageButton)
img.BorderStyle = BorderStyle.Ridge
SortOrder = e.CommandArgument
BindData()
Catch exc As Exception
ProcessModuleLoadException(Me, exc)
End Try
End Sub

My problem is that by clicking on the image button in the gridview header, the OnCommand event is not being fired. I have tried this with both OnClick and OnCommand and neither fires. I can't see what I am doing wrong if anything.