I have one site that has been in the rich snippets program since the beginning while another which was very similar never got picked up, despite perfect results in the Rich Snippets Testing Tool.
I decided one day to just redo the entire structure of the code and within a day, rich snippets were showing in the SERPs. Both versions were perfectly structured.
I believe one thing than may have affected it was placing data within divs instead of spans. Who knows though.