The seatpost on my new Brompton kept slipping even after degreasing everything and tightening the quick release nut. I took it to a local dealer who replaced the seat sleeve and reset the quick release nut so that it wasn’t as tight as its allowed to be. That’s great, because the seat sleeve wears out over time and this means I’ll have latitude to tighten things up as that happens.
Here’s the old one! They’re glued into the main frame.
It slipped a little riding it home but I think it’s totally fixed now after degreasing the seatpost and sleeve once more, and tightening the nut just a bit.