А у тебя QGroupBox чекабленный?
Клик посылается только в этом режиме, и только при нажатии в чек:
void QGroupBox::clicked ( bool checked = false ) [signal]
This signal is emitted when the check box is activated (i.e. pressed down then released while the mouse cursor is inside the button), or when the shortcut key is typed, Notably, this signal is not emitted if you call setChecked()
.