QML object property changed from c++, but I can't see the result

So I could change the property of a certain QML object via C++ code, but I couldn't see the result on screen. I have an item repeated 64 times, and I want a certain image to be displayed only for the 32nd item (from C++) so I used invokeMethod to access the object via C++ then I used setProperty to change the visibility, if I view it with qDebug the property "visible" did change, but I notice no difference on the screen I still cannot see the image, but if I change the visibility from qml, I can see it.

This is the C++ code:

int main(int argc, char *argv[])
{
  QGuiApplication app(argc, argv);
  QQuickView view;
  view.setSource(QUrl("qrc:///main.qml"));
  view.show();
  QQuickItem* child;
  QQmlApplicationEngine engine;
  engine.load(QUrl(QStringLiteral("qrc:///Board.qml")));
  QObject *rootObject = engine.rootObjects().first();
  QQuickItem *qmlObject = rootObject->findChild<QQuickItem*>("grid")->findChild<QQuickItem*>("repeter");
  QMetaObject::invokeMethod(qmlObject,"itemAt",Qt::DirectConnection,   Q_RETURN_ARG (QQuickItem*,child), Q_ARG(int,32));
  child=child->findChild<QQuickItem*>("pleaseWork");
  qDebug() << child->property("visible");
  child->setProperty("visible","true");
  qDebug() << child->property("visible");
  return app.exec();
}

I used qDebug to verify the property changed

This is the QML code :

Item 
{
    id: root
    width: 8*45
    height: 8*45
    Grid 
    {
        id: grid
        objectName: "grid"
        rows: 8
        Repeater 
        {
            objectName: "repeter"
            model: 64 
            Image
            {
                objectName: "test"
                width: 45; height: 45 
                source: "images/dark_square.png"
                Image
                {
                    id: isit
                    objectName: "pleaseWork"
                    visible: false
                    source: "images/avail_dark.png"
                }
            }
        }
    }  
}