[Plugin: WordPress SEO by Yoast] Not showing excerpt in snippet

WordPress version: 3.3.1
WordPress SEO version: 1.1.5

I did this:
Recently improved the site with more focussed settings in WordPress SEO and installed breadcrumbs for hierarchy. Site has been running for 3 months. SEO post settings are: %%title%% for title, %%excerpt%% for snippet and %%focuskw%% for meta-zoektermsjabloon.

I expected the plugin to do this:
I expected the plugin to create snippets according to the excerpt, like it had done up until the last Google crawl.

Instead it did this:
Since then, Google has created its own, very irrelevant snippets. It even shows the coupon codes in the snippets (it’s a coupon site), so you can see this is not what I want.

I’ve also checked the header.php and read about rich snippets, but this is not the case here. Hope you can help me out here.

https://www.ads-software.com/extend/plugins/wordpress-seo/