XPS 13 (9350) Tablet Mode is enabled but not working?

Hi to all XPS 13 experts, my problem is:

nothing happens after enabling the tablet mode (Windows 10)... Touching the surface nothing happens... why?

further details to my config:

- system bios is current (v1.1.9)

- all windows 10 updates are installed

-  all dell updates are installed

- dell diagnostics quick test successfully completed  without any errors

? is there any hidden bios option, which has to be enabled???

Many thanks for every answer! robert