CollapsiblePanelExtender in Custom Server Control

 Hi!
 
I'm writting down a custom server control that has a collapsible panel extender, the problem is that when i'm testing the control it's always expanded, when i try to collapse it it stays the same way (expanded. The code is in spanish sorry, i'll explain in english relevant parts of the cod-
 Here's the code.
        // ATTRIBUTES

        private CollapsiblePanelExtender CollapsiblePanelExtenderBuscador;

        private System.Web.UI.WebControls.Image ImageExtendidoEncojido;

        private Label LabelTituloPanelBusqueda;

        private TextBox TextBoxBusqueda;

        private ImageButton ImageButtonBuscar;

      &n ...