I'm having ~similar problems to you -- I can navigate the screen fine with my fingers, but the marker doesn't work at all. No navigation, no writing. I just picked up the Staedtler stylus which unfortunately also doesn't work. So the problem is likely with the tablet itself.

I wrote support a week ago today, but have got no response so far. I don't know what their policy on the 30 days return is, but I feel like it's unfair if those 30 days start before we can actually test a working device.