The perennial folding bike issue. It happened with my Dahons until I greased the contact area between shim and frame. I assumed that Bromptons wouldn't suffer from this problem as they're talked about as though they're engineering marvels, hand-built to perfection.

Nope, same problem occurs and there's some sort of crappy plastic shim that's glued to the frame, so no opportunity to grease between the two for my go-to Dahon fix. Looking at the contact area between seat post and shim when clamped there's a clear gap in certain areas, i.e. the shim doesn't contact the post around it's entire circumference (as would happen if the shim had some freedom to move independently from the frame, and to slide as the clamp were shut). The Dahon design is superior.