mvvm-light RaisePropertyChanged messenger not broadcasted

Question

I cant seem to get this working properly.

I've got two ViewModels. VM1 registers a message with the following code

Messenger.Default.Register<PropertyChangedMessage<int>>(
    this,
    (action) => DispatcherHelper.CheckBeginInvokeOnUI(() => UpdateOverlayIcon(action.NewValue))
);

VM2 fires the RaisePropertyChanged when property is being set.

// Update bindings and broadcast change using GalaSoft.MvvmLight.Messenging
RaisePropertyChanged(UnreadTweetsPropertyName, oldValue, value, true);

Now, shouldn't the VM1.UpdateOverlayIcon() method run right after the RaisePropertyChanged is fired in VM2? Or have i missed an essential part of this, because it doesn't work for me? I have been struggling with this all day long and tried a number of different methods.