If your dropdown is in the update panel then the dropdown selectedIndex will not fire until you used Update panel trigger.
A sample code is attached. try to make it. if still have problem. post the code.
<asp:asyncpostbacktrigger controlid="DropDownList1" eventname="SelectedIndexChanged" xmlns:asp="#unknown" />
Use this trigger inside of your update panel Modal popup.
write your dropdownName in the control ID and then use the selectedIndex.