DropDownList and CheckBoxList ViewState losing value after button click

Hi, I have a DropDown and a CheckBoxList that loses their value when I click the button to submit. Any help please?

<asp:DropDownList ID="ddlFilterCtx" runat="server" EnableViewState="true"><asp:ListItem Selected="True" Value="">Choix Ctx</asp:ListItem><asp:ListItem Value="">Choix Ctx1</asp:ListItem><asp:ListItem Value="">Choix Ctx2</asp:ListItem><asp:ListItem Value="">Choix Ctx3</asp:ListItem><asp:ListItem Value="">Choix Ctx4</asp:ListItem></asp:DropDownList><asp:CheckBoxList ID="cklFilters" runat="server" EnableViewState="true"></asp:CheckBoxList><asp:Button ID="btnVisualiser" runat="server" Text="Visualiser" onclick="btnVisualiser_Click"/>