Problem with Dynamic DropDownList

Background:  I have a group of  radio buttons.  Depending on the selection, a dropdownlist will appear displaying the results of a query.  There are 2 different queries.  The code is fired from the Page_Load event.  Here it is:
//When user selects a PROBLEM SOURCE... switch (radioProbSourceType.SelectedValue) { case "Hardware/OS": //make the hardware dropdown list appear and enable the validation srcTechInv.SelectCommand = "select c.cc_name + ' ' + d.devtype_value + ' ' + s.sys_name as HardwareName from tblsystem as s left join tblludevtype as d on s.devtype_id=d.devtype_id left join tblcostcenter as c on c.cc_id=s.cc_id left join tblluos as o on o.os_id=s.op_system where s.devtype_id is not null order by cc_name, devtype_value, sys_name"; ddlistAppName.DataTextField = "HardwareName"; ...