Vista - dropdownlist databinding problem

I have a radiobuttonlist control which dictate what to display in a dropdown
list:

<asp:SqlDataSource ID="sqlDataSrouce1" runat="server" ConnectionString="<%$
ConnectionStrings:ConnectionString1 %>"

SelectCommand="SELECT * FROM [qry2] ">

</asp:SqlDataSource>



Protected Sub rdlEventType_SelectedIndexChanged(ByVal sender As Object,
ByVal e As System.EventArgs)

If rdlEventType.SelectedValue = "abc" Then

sqlDataSrouce1.SelectCommand = "SELECT * FROM [qry1]"

Else

sqlDataSrouce1.SelectCommand = "SELECT * FROM [qry2]"

End If

dropdown1.DataBind()

dropdown1.SelectedIndex = 0

End Sub

The radiobuttonlist has one of the non-"abc" buttons checked as the
default.

Upon entering the form, I select "abc" first. The dropdown displays data
from qry1 correctly. Without exiting the form, I switch to a a non-"abc"
radiobutton. The dropdown then displays qry2 data fine. Then come the
problem: if I switch back to "abc", the dropdown STILL displays qry2 data.
So why doesn't the data binding work the third time?

I also tested having the SelectCommand setting removed from the
SqlDataSource control itself. No luck; the behaviour is still the same.

Any insight appreciated.