craftgrrl | Dress Fitting Problems

I am making a mock up for the support layer of a dress and am having real problems with the fitting round the bust area. I have drawn a crappy diagram for you below!
The pattern is adapted from the contour block in Patternmaking for Fashion Design. It would fit perfectly if i could get the centre front seam to stay in place! Instead of laying flat against my chest it gaps out so the whole thing hangs loose. The red lines on the diagram show how it fits (top down view) and below how it should fit.
Is there something I am missing in how to construct this? Its made from cotton duck (i think? its a heavy cotton fabric) and boned along the seams. There is also some grosgain ribbon sewn along the top edge which sort of helps but not much.
Sorry I have no real photos my camera is killed.
Thanks very much!