Open Graph tags are not working
I've studied this a lot and didn't managed to make it work with the Open Graph tags. Because they have to be on the head part of the page and you can have a hold of the post variables in the body.
Open Graph from the raw tags
So the next variant is to let Facebook guess where is the description from the raw tags.
Unfortunately the "guessing algorithm" is not documented.
See it for yourself on their debugger: https://developers.facebook.com/tools/debug/
Happened to me, while their debugger showed there is computed a description, on the actual share the description wasn't there. And no way to test it.
On the other thoughts, Facebook reads the pages once and not on every click of the share button, therefore if there was no description computed when their scraper read the page, and its added later it won't appear until their cache is invalidated.
Make sure you added the description before the post is published.
For my situation I have changed the share buttons in the the template. For Facebook I have used the feed API and constructed the share URLs like this: