Until recently, the pivoting rear frame of a Brompton folding bike (which is an both an essential and iconic part of the design) didn't latch onto the bike's main frame leaving the rear section to dangle like a numb limb any time the bike was lifted.
Modern Bromptons (those built after about August 2007) have a catch at the back of the main frame, integrated with the seat post quick release clamp, that hooks into the bolt securing the suspension block to the rear frame.
Occasionally the latch fails to click in - which can be quite disconcerting! So here are a couple of reasons why your rear frame might not be locking in:
Vibrations experienced in the normal course of transporting your Brompton commuter bike may result in the suspension block twisting. To correct it's just a simple case of twisting the block back around so the notch is back at the bottom (in the 6 o'clock position).
Following on from the above, you may need to check how tight the nut is securing your suspension block bolt. If your suspension block feels wobbly you should attend to this right away. Tighten the nut by turning clockwise with a 10mm spanner so that about 1.5 threads are protruding from the end of the nylock nut. Don't over-tighten.
Flat Rear Tyre
Yep, really. Sometimes it just won't click in because there's not enough resistance from the rear. Fix your flat (or keep your tyre pressures within the recommended range) and it should be fine again.
Worn Latch or Bolt
Parts wear with repeated use. Depending on how often you park or fold your Brompton, you may find that you've worn out the surfaces of either (or both) of the latch or the bolt. Luckily it's an easy fix and something that you can do yourself at home, or have included with your next Brompton service.