Hi Marine,
By default, a ctkCollapsibleButton comes without a layout. So you are experiencing the correct behavior.
To set the layout, you should not use the Layout item in the left panel of Designer, but instead, you need to right click your ctkCollapsibleButton and select “Lay out” in the context menu.
Hth,
Julien.
p.s. if that still does not work, you can always copy/paste the existing collapsible button. And then remove its contents.