I was encountering the same problem when using a WordPress page as my site’s home page.
To fix this you need to add a custom page excerpt, similar to the post excerpts. Although wordpress pages can support excerpts (they’re accounted for in the database) it hides the text field to edit the page excerpt by default.
To activate the page excerpt field go to your home page editor and click screen options at the top right (next to help) then click the excerpt check box.
Now you can add a custom excerpt to pages which this plugin will pull for the og:description.
