I have two Hope seat clamps which work fine. I don't particularly like the alu on alu screw, but they are tarty. What is your frame? Any seatclamp should work if the frame has been made to tolerance. Also, have you lined up the slot in the frame with the slot in the seat clamp?
Well... had to find it out the hard way I guess... their clamps seemed rather nice. Nicely machined, oversized clamp-bolt...