Forms Data Controls :: DropDownList.SelectedIndexChanged Not Working

I have the following in a web user control:

asp:DropDownList
ID="ddlPrescriptionCodeGroup"
runat="server"
AppendDataBoundItems="True"
AutoPostBack="True"
DataSourceID="PrescriptionCodeGroupDataSource"
DataTextField="CodeGroupDescription"
DataValueField="PrescriptionCodeGroupID"
onselectedindexchanged="ddlPrescriptionCodeGroup_SelectedIndexChanged">
<asp:ListItem
Text="--Please Select--"
Value="-1"></asp:ListItem>
<
</asp:DropDownList>
<asp:SqlDataSource
ID="PrescriptionCodeGroupDataSource"
runat="server"
ConnectionString="<%$ ConnectionStrings:RadiotherapyConnectionString %>"
SelectCommand="SELECT [PrescriptionCodeGroupID], [CodeGroupDescription] FROM [PrescriptionCodeGroup] WHERE (([Archived]
= @Archived) AND ([OrganisationID] = @OrganisationID))">
<SelectParameters>
<asp:Parameter
DefaultValue="False"
Name="Archived"
Type="Boolean"
/>
<asp:SessionParameter
Name="OrganisationID"
SessionField="OrganisationID"
Type="Int32"
/>
</SelectParameters>
</asp:SqlDataSource>
<asp:DropDownList
ID="ddlPrescriptionCode"
AutoPostBack="True"
runat="server"
AppendDataBoundItems="True"
onselectedindexchanged="ddlPrescriptionCode_SelectedIndexChanged"
DataSourceID="PrescriptionCodeDataSource"
DataTextField="CodeDescription"
DataValueField="PrescriptionCodeID"
>
<asp:ListItem
Text="--Please Select--"
Value="-1"></asp:ListItem>
</asp:DropDownList>
<asp:SqlDataSource
ID="PrescriptionCodeDataSource"
runat="server"
ConnectionString="<%$ ConnectionStrings:SystemConnectionString %>"
SelectCommand="SELECT [PrescriptionCodeID], [CodeDescription] FROM [PrescriptionCode] WHERE (([Archived] = @Archived)
AND ([PrescriptionCodeGroupID] = @PrescriptionCodeGroupID))">
<SelectParameters>
<asp:Parameter
DefaultValue="False"
Name="Archived"
Type="Boolean"
/>
<asp:ControlParameter
ControlID="ddlPrescriptionCodeGroup"
Name="PrescriptionCodeGroupID"
PropertyName="SelectedValue"
Type="Int32"
/>
</SelectParameters>
</asp:SqlDataSource>

When I select from the first dropdown I want to populate the second. However, this is not happening. The postback is happening, but the SelectedIndexChanged handler isn't being called (I have a breakpoint which isn't being hit). The user controls are created dynamically and put into a PlaceHolder which itself is within an AJAX Updatepanel, with ChildrenAsTriggers set to true.