I think you need to distinguish between the body language you have taught the dog to display, and the concept in her mind of what she's supposed to be doing. You've got a good response from her on the first bit and a poor one on the second because she doesn't know whats expected of her.
So in no way is the dog "lying", quite the opposite, she's telling you the blunt truth "You haven't taught me well enough". Once you've grasped that you can go forward with the suggestions the other guys have made.
In teaching the concept of a dead cold completely un-cued blind (no shot, no cones, posts, and in new terrain) I like to make use of a rise in the ground just a few yards from the send out spot, that conceals the dummy from the dog's level of sight. Once it breasts the rise, it sees the white dummy standing out like a beacon giving a great addition to momentum.