That’s how Google+ works. It only includes the Post Title and the Post URL. This is true even with other themes, you can check by looking at other people’s Google+ Profile or by changing themes then sharing a post.
You could try asking the author of the sharing plugin you’re using. I don’t think there’s a way to automatically do this yet as I have not seen any Google+ Profile with a shared post that has an excerpt in place of the url. However, you can just manually copy the excerpt to the “Add a comment” section.