[Plugin: Jetpack] Can not get jetpack comment to work

I am using the latest wordpress and jetpack plugin. I m trying to allow ‘facebook’ style comment on a wordpress blog. I changed the ‘greeting message’ but the blog is still using the old comment style. I read the following article which says the theme needs to use comment_form() function to display the comment box.

https://jetpack.me/support/comments/troubleshooting-jetpack-comments/

I check the comments.php from the theme, and I see the line

comment_form($comments_args), which I assume means it is using the right function. Does anyone have any idea what else I should check? I have no experience in php but I do have some basic programming background.