Calligraphy brush gets uniform insteat of varying thickness after painting

Hi

I started with a new Document, create a new Calligraphy Brush, setting Thickness to "pressure" instead of "fix", Line Thickness 0 - 9pt.

While painting with that brush everything looks fine (thin line at light pressure, thick line at heavy pressure).

But instanty after finishing the line it get's a uniform thickness.

Any idea what I do wrong ?

PS.

Illustration newest verion

Surface Pro 4 with Pen

Windows 10