UpdatePanel，其中按钮事件不会触发

I have a UpdatePanel with a PlaceHold contained in it. I create some controls with Labels and Buttons, when Button is clicked it fires an Event that clears PlaceHolder and adds some Textboxes and a Button with an Event. Problem is when this Button is click it appears to do a PostBack and does not fire Event associated with Button. I thought that since these controls are all contained within the UpdatePanel the would be no PostBack, am I missing the flow.I have a UpdatePanel with a PlaceHold contained