I think the firewall is turned on. Turn it off! And sometimes it doesn't work for me too...then I just restart the DS and the senpkmn tool. Then it works.
Uhm, I've got a problem. I have a German Pearl (but I'm not from Germany), and I can't connect to the server. I tried with my HG too and didn't work. Should I try again with my Platinum or with my Pearl until it works? Thank you.