Just because you have the code correct doesn't mean Rich Snippets will appear in the SERPS. Many are in the same boat as you.
In my case the Rich Snippets did appear but six months or so they disappeared with no reason I could see. But then again, many Rich Snippets, mine included, were really a pile of junk. I didn't want G to have too much of a handle on my content. Possibly loads of other people were doing the same which led to the withdrawal of them in the SERPS for so many.
Schema markup is a form of meta data, which may be necessary to help Google identify material suitable for display as rich snippets... but by itself this markup is not sufficient for rich snippets to be displayed. If you want rich snippets, there are situations where you are going to have to use the markup, like it or not, but Google also weighs other quality signals before it displays the snippets.
In my case the Rich Snippets did appear but six months or so they disappeared with no reason I could see.
Matt Cutts mentioned at PubCon last year (in Oct 2013) that rich snippets would be removed from roughly 15% of the lowest quality sites. From the timing, it seems like that's what hit you. Now that the bar has been raised, it will probably stay up there.
See longer discussions in these threads, among others...