Since the obelisk changes every 2 hours, and only stays active for 20 minutes, it is most likely that there is no obelisk active at the time. The best way to find out when an obelisk is active is to open the Adventures interface and select the Minigames tab, from here players can select D&Ds from the drop down menu and see in real time when the next obelisk event will be active.
I am having the same problem. I check the minigame tab and as soon as it SHOULD have an obelisk (at 20 minutes left on countdown) i teleport and talk to pikkupstix but he says he's busy and to come back later, every time. I've gone every two hours and sat there for 20 minutes straight for the last two+ weeks. Please Help!