dropdown box selectedindexchanged event doesn't fire problem

Hi All,
I have problem to make the dropdown box selectedindexchanged event to work. I have no problem with other applicaitons before.
This is a search form, I want to display a radioboxlist control on the same form when user selects a certern value from the dropdown box otherwise the radioboxlist control is invisable. But the code does even goes to the selectedindexchanged function.
The dropdownlist control autopostback is set as false and the enableviewstate is true. TIA.
 
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