Qt: QMenu does not work on Linux

Qt: QMenu does not work on Linux

I have added a QMenu to QMainWindow like this:
boost::scoped_ptr<QMenu> file_menu;
file_menu.reset(menuBar()->addMenu("&File"));
file_menu->addAction(my_action1.get());
file_menu->addAction(my_action2.get());
...
And it works perfectly on Windows but not on Linux. The "File" menu
appears on the window but nothing happens if I click it.
The Qt version is 4.7.3 and I use Ubuntu 11.04 via VirtualBox 4.1.2.