Page Load of User Control Not firing inside another User control

Hi,

   In my application, I have one user control say UC 1 inside that I have another User Control say UC2. In UC2 there are few DropdownList which I fill in !Ispostback section. Now when I run my application, This !Ispostback section not fire hence dropdownlist wont get filled. I want to fill all dropdown list on load. While debugging control always goes in else part and DropDown List won't get filled.

Code:
           Page_Load of User Control 2

   if (!Page.IsPostBack)

   {
   // This part never get called. Why??????
   FillDropDownList();
        }
   else
   {
   //Do something...
        }

Any help appreciated, Thanks.

Regards,

Vikram