I just got a Tankstick a week or two ago. I had the same problem with the right stick. I opened it up, and the four levers that are attached to the microswitches each had a slight bend. I called tech support, and Emailed them a pic, and they got replacements sent out right away. They really do support their products as advertised. Replacing it fixed the problem. I will admit though, it was kinda nice while it was like that, as I would play 8way games on the left and 4way games like Pac-Man, Donkey Kong and Mr. Do on the right. Kinda sucked for Robotron though.
