Xamarin Forms Previewer isn't showing

Like @jgoldberger said, the XAML previewer is still in early development. However if you are on the Xamarin Stable update channel it might be worth switching to the Beta update channel.

There have been vast improvements in the XAML previewer in the next release, which is currently on the 6th RC iteration (ie. very close to stable). This way you can at least see if the previewer improvements fix your issue. You can always return to the Stable update channel if you choose.