Create Control in aspx.vb and Pass to Page

I'm having trouble in displaying a dropdownlist on an aspx page when it is created in the codebehind.
The aspx (addCase.aspx) page has a dropdownlist called ddlCaseType
<asp:DropDownList ID="ddlCaseType" runat="server"></asp:DropDownList> I have a class that contains a function which creates a dropdownlist control, populates it and returns it.
The codebehind (addCase.aspx.vb) calls a routine at page_load which calls the routine to create the dropdownlist and then sets the value of the dropdownlist on the age to it.
Private Sub fillCaseType() Me.ddlCaseType = Case_Type.getCase_type_dropDownList End Sub When I open the page, the dropdownlist is empty., even though I have confirmed that the dropdownlist has rows, both at creation and after it has been assigned to the control on the page.

Any ideas?