RaisePostBackEvent never fired

[ASP.Net 3.5]
I have built a composite control. In the CreateChildControls overload I add a new button server control to the controls collection. I also save the postbackreference for that button to be available to client script.
<snip>
Page.ClientScript.GetPostBackEventReference(_updateTrigger, "propertiesChangedEvent"); 
I also register  the control for event validation
<snip>
protected override void Render(HtmlTextWriter writer)
        {
            Page.ClientScript.RegisterForEventValidation(_updateTrigger.UniqueID, "propertiesChangedEvent");
            base.Render(writer);
        }
 
But, the IPostBackEventHandler.RaisePostBackEvent never fires on this custom control. However, I am able ...