Could be that bolt is bottoming out on the "free" end before the free end is able to tighten up onto the 'keyed' part of the frame. This happened when I replaced a similar binder bolt on my wife's old Bianchi. The new bolt was too long and was bottoming out, and I wasn't able to get it tight enough to hold the seatpost in place. I had to use some washers to solve the problem.