so I have installed the Yoast WordPress SEO plugin on my site, I have everything setup correctly (or so I think) and I enter a meta description for every single post, however the snippet in Google is showing the first lines of the post.
I even checked the page source of any article and the meta descrition is correct and there´s only 1 definition of it.
I also checked the header.php and there´s nothing creating a second meta description, so it can´t be the theme (right?)
If Google had already crawled your site before you added the meta tags, then the snippet they show will be just from your content. Google crawls all sites differently, so it might take some time for them to pick up your new meta tags.