The first of my 2012 girls laid her first egg out by the waterer. I think the egg was a surprise to her (and why wouldn't it be??). After that she hit the nest as have all the "first timers" since.
Have your girls checked out the nest boxes at all? Along with Kurt's questions about the accessibility, etc, you could try putting them in the nest box (assuming you don't have to traumatize them) so they can see the possibilities. Especially since it is obviously a safe place to lay, the fake chicken already laid a fake egg in there to show that
Our first egg was outside just sitting in the middle of the grass, the rest have just been randomly laid on the coop floor.
They have been checking out the nest box for over a week, climbing in and out easily, but then just sit on top of it, never actually lay in it.
These are my first ever chickens, so I really thank you all for your suggestions - going now to google up nesting box curtains!!!