Mutually exclusive radiobuttons in Custom Controls

Hi all, im having a bit of a problem here, im trying to get radiobuttons to be mutually exclusive, but i cant seen too do it. Here�´s the scenario:

I have a PlaceHolder, inside of it im loading multiples custom controls (ascx) dynamically. Each control has multiples controls in it, but has only 1 radiobutton.
Now, i want to be able to select a radiobutton, and that the previously selected one gets Unselected. I�´ve tryed setting the GroupName on the ascx radiobutton in the ascx control, didnt worked.
  
1 <asp:RadioButton ID="rbtnIDNivel1" runat="server" AutoPostBack="True"
2 GroupName="Nivel1"/& ...