why my selectedindex_changed event not fired

protected void Page_Load(object sender, EventArgs e)
    {
       Loademployeegeofencee(strUnitNo);
    }


private void Loademployeegeofencee(object strUnitNo)
    {
        //here i load the value from the database to the dropdownlist "Superuserid1"
        //this works fine
        
    }

protected void Superuserid1_SelectedIndexChanged(object sender, EventArgs e)
    {
              //code to be executed

    }