There are some common pitfalls in GTK programming.
When a GTK widget is created it will be hidden by default. In order to show it, a call to gtk_widget_show() has to be done.
It isn’t necessary to do this for each and every widget created. A call to gtk_widget_show_all() on the parent of all the widgets in question (e.g. a dialog window) can be done, so all of its child widgets will be shown too.