In reply to jon:
Yes, that's my experience of them. I've had two of them, and same problem with both hence why I switched to a home-made one and have never looked back. It always seems to start on the smallest section.
I would say maybe I'm just not careful enough with them, but I've borrowed several of them at various crags over the years and never found one with a working twist-lock for the top bit.
Perhaps I've just been unlucky!