下拉列表selectedIndexchanged事件不会被触发

I have page that contains four dropdownslists on a page in updatepanel and a button.I am binding data to all dropdowns on Page_Load event and when i select item from any one of dropdowns their respective selectedindexchanged event does not get fired at all.I have set Autopostback property of all dropdowns to true.This makes page_load event to call but does not fire selectedindexchanged event.Below is my markup;I have page that contains four dropdownslists o