User control

I have a user control on which i using two dropdownlist . second one binding on the basis of first.
I am using this usercontrol in the item template or edititem template of the datalist on a page.
But when i binding the data , its raise an expection that the particule dropdownlist does not contain that value which i am assigning. but i m assignig the same value which it have......


The reason my be that the usercontrol in not doing postback and not loading to bind the data in dropdown list.


Please Help . and provide ur opinion abt this what i have to do?