Activity Indicator

[VBusch-netsol]

Hy,
i have an activity indicator on my page:

I can show it with a button, that works:

In Viewmodel:
public void ShowBusy()
{
if (IsBusy)
IsBusy = false;
else
IsBusy = true;
}

I have a function in the viewmodel which needs longer, but when i start the function with IsBusy = true; and end the function with IsBusy = false; the BusyIndicator is not shown.
also not when i start the function in an other function:
private async void ShowBusyIndicator(string pdfFilename)
{
IsBusy = true;
bool finished = await GetAndShowPDF(pdfFilename);
IsBusy = false;
}

What do i wrong?

Volker