DropDownList is not fireing SelectedIndexChanged Event

[Dave]

I've got many forms that use this SelectedIndexChanged Event on drop down
lists, and they all work fine... I don't know what I did wrong this time
I've even tried copying and rebuilding the page. When it posts back it
fires the "Page Load" procedure but never continues to fire the
SelectedIndexChanged event. Is there something basic I should look for? I
have Autopostback=true, and I have made sure the Event handler was added to
the InitializeComponent() procedure in the Web Form Designer region.

<asp:dropdownlist id="ddlVendorNo" Runat="server" Width="100%"
AutoPostBack="True" />

this.ddlVendorNo.SelectedIndexChanged +=
System.EventHandler(this.ddlVendorNo_SelectedIndexChanged);

private void ddlVendorNo_SelectedIndexChanged(object sender,
System.EventArgs e) {}