FWIW, I've found G's previews+snippets 'feature' (which I generally like when it works) to be very, and very puzzlingly, inconsistent...
Re-searching for my sites by domain, one had scores of pages with previews the other day, but yesterday only a handful of pages appeared at all (regardless of the "More results from..." and "Show all results from..." options). Another very small site shows no preview icon for its home page, yet there's one on an inside page, and again all other pages don't appear at all despite GWT stating all are indexed. A third site shows previews for all pages except two, even though all have the exact same parameters. Yet another site's pages, with or without previews, also come and go completely.
Yeesh. Talk about crazy-making!
I finally decided to stop checking those sites and more because there are no clear reasons why things come and go -- aside from that old 'propagating across servers' saw -- and there's apparently little I can do anyway. (Also, refreshing robots.txt files and re-submitting sitemaps to GWT makes no difference.) So all the best with what you see in your SERPs, and when and where and how anything appears at all.