[Plugin: Jetpack by WordPress.com] Comment form missing

I have two independent websites, my own and one owned by someone else who occassionally asks for my guidance – having the same issue. Comments forms are randomly appearing and disappearing, depending on the post. Several of our readers have pointed this out.

I’ve checked error logs for something unusual, but found nothing. There aren’t any errors on the page. There don’t appear to be any JavaScript errors, either.

Any guesses?

http://wordpress.org/extend/plugins/jetpack/