not all application windows show after leaving and returning with ALT/TAB

I have made this very simple sample:

In your MainWindow.xaml

<Window x:Class="SampleDialog.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Content="Show Dialog" Click="ButtonBase_OnClick"></Button>
    </Grid>
</Window>

In codebehind

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        {
            new Dialog(this).ShowDialog();
        }
    }

In Dialog.xaml

<Window x:Class="SampleDialog.Dialog"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Dialog" Height="300" Width="300" 
        Activated="Dialog_OnActivated" Deactivated="Dialog_OnDeactivated">
    <Grid>

    </Grid>
</Window>

And in Dialog codebehind

public partial class Dialog : Window
{
    public Dialog()
    {
        InitializeComponent();
    }

    public Dialog(MainWindow main)
        : this()
    {
        _main = main;
    }

    private void Dialog_OnActivated(object sender, EventArgs e)
    {
        Topmost = _main.Topmost = true;
    }

    private void Dialog_OnDeactivated(object sender, EventArgs e)
    {
        Topmost = _main.Topmost = false;
    }

    private readonly MainWindow _main;

The intention is to handle this functionality in Activated and Deactivated events on your modal window.

Hope it helps.

(If you are using MVVM then you must refactor to handle this on a WindowService class or by using EventToCommand approach)

EDIT>>> This works for your case when ShowInTaskbar = "False". =)