It seems that the excerpt are not been taken from the post content of those specific posts where the content is not showing. There might be improper up-gradation of the Cornerstone plugin is one of the reasons behind your issue.
I would also suggest you follow the below steps and update the Cornerstone once again.
- Delete the Cornerstone existing one
- After deletion please use the link in the dashboard to automatically install the Cornerstone.
If that does not resolve the issue, please copy your live site to a staging server so we could troubleshoot freely without breaking your live site.
And give us access in the secure note including:
– WordPress Site URL & Login URL
– WordPress Admin username/password
To create a secure note, click the key icon underneath any of your posts.
You can also fix this by adding the Manual Excerpt from the Post > Settings, as explained in the screenshot.