I've just done a small test. I can start a new thread on my own MN Local (not that I did), but I also tested on a completely different area and I could also start a new thread there - and it still uses my MN Local NN.
Sorry to hear you've been having trouble posting a thread on Local.
We've checked your registration details and everything appears in order, so we're not quite sure what's going wrong here. Could you please drop our Local team a line at firstname.lastname@example.org so they can help you out?