drop down list postback

Hi
I load a drop down list using an add
DDListPrefernceOne1.Items.Add(SubCat);
I select one of the adds

DDListPrefernceOne1.SelectedValue = SubCat;
When the user changes the selection I expect a selected value would give me the selected value.  It gives me the old one.

Preference1 = DDListPrefernceOne1.SelectedValue;
I have tried auto  post back and many other things. 
There must be a simple solution to this problem.
Thanks
Herb
 
 
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