Will eventually try with a different clamp. In the meantime: the seatpost isn't positively locked in place, despite the bolt being properly tightened. The frame is a Surly Troll. The seatpost is a Nitto, 27.2mm. The clamp is stock.

There is some grit on the clamp, but it doesn't seem to interfere. As we are currently traveling, I don't have solvent on hand, so we adjust frequently.

Any idea what could be the cause??