Modal Dialog not displaying when trying to launch Second instance of application on Windows 8.1

I have a C# .net (4.5) Desktop application. I am trying to display a modal dialog (warning dialog) when ever user tries to launch second instance of application. It is Working in all cases but one case.

Apparently when second instance is launched from win 8.1 launcher , neither dialog does appear nor the application is launched.

I am using Messagebox.show() to display the dialog.

Is this a behavior ?

Any help is hugely appreciated.