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??

I found this answer from a google groups search for someone who had the same problem on a different Surly.

"The issue was not the seatpost, it was the seat post clamp. I brought
the bike into my LBS and right away the mechanic thought it was the
seat post locking collar. So we found a Cross Check on the showroom
floor that had the large, super beefy Surly constrictor seat post and
put that on. That was the magic trick! I am a heavier rider (210 lbs)
so I imagine with my weight, riding mostly gravel, it needs just a
little bit more bite around the seat post. I’ve got the small seat
post collar from Surly on my LHT and have had no issues what so ever,
but that may or may not have something to do with the fact that I ride
my trucker on typically smoother surfaces that my KM."

I would remove the seatpost and the clamp from the bike and clean it and put little chain lube on the clamp and see what happens.

Thanks for the info. Makes sense. The literature suggests that the Troll's clamp should be stainless steel (it is on my LHT) but it isn't.

Another thing to do (along with cleaning and greasing everything) is to ensure the gap in the clamp is aligned with the gap in the seat tube:


If that doesn't fix it you can make a shim from an aluminum can or get the Surly heavy duty clamp.

now even Surly's stainless steel band clamp uses a self aligning barrel nut.

The aluminum one (Salsa is another QBP brand name) has that kind of barrel nut on it Right?.