I have a USE SUMO suss seatpost, bought to replace an earlier model that was lost. The earlier model was fit and forget. The new one has been tightened to the specified torque more than once, but has a habit of slipping backwards.
I plan to re-assemble with grease on the mating surfaces this time, and see how that works, in case it tightens more effectively. The step beyond this may be a new longer bolt and a bigger nut and washer.
Has anyone else had this problem and found an answer.