ASP.NET - Can't get SelectedIndexChange to fire.

I have a DDL on a standard ASP page:

   <asp:DropDownList ID="ddlReportCategoryID" runat="server" Style="width: 370px; float: left;">
   </asp:DropDownList>

Select allOpen in new window

I've attempted to associate it with with the SelectedIndexChanged event:

   Protected Sub ddlReportCategoryID_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles ddlReportCategoryID.SelectedIndexChanged
      MsgBox("hello")
   End Sub

Select allOpen in new window

No matter what I do, I can't get the code to fire:

- I tried making a new DDL with a couple ListItems.  SelectedIndexChanged didn't fire.
- I tried placing the DDL on a new page.  SelectedIndexChanged didn't fire.

All other ASP on the page seems to be working just fine.