WPF Topmost doesn't work as expected.

Hi,

I have two Topmost app, if I run it 20 times, it might fails 1-2 times. I don't know why it doesn't work?

Have you encountered with this experience before? .NET 4.0

Title="OnScreen" WindowStyle="None" Background="Transparent"
        WindowState="Maximized" ShowInTaskbar="False" AllowsTransparency="True"
        Topmost="{Binding Path=IsTopmost}"

Thanks,

weera