That is a known issue with the helmet mount. Mine hasn't been a problem for some months now.
The way you say "for some months", leads me to the conclusion that you did previously. What changed?
Ah, so I'm not the only one then. At least it's something that someday may get rectified. It just seems to me that the metal rod is a tad too short. If you were on a pretty rough surface on a road bike, I dare say that without modification, you'd end up losing the rod and having your ContourHD hanging from the lanyard after not too long.
I'll see if I can come up with something a little more suitable than pieces of folded paper to stop the rod sliding out.
Gaz, what would you use instead of bits of paper? A piece of plastic, like a piece of an old expired debit card or something?