WP comment checkbox isn't showing

Hi there,

The new WP checkbox that should appear under the comments box "Save my name, email and website..." is not appearing under my comments. I've checked on two browsers (Edge and Opera) that I don't use to log-in to my website and where the cache is empty.

WP suggest (here: https://wordpress.org/support/topic/read-this-first-wordpress-4-9-master-list/#post-9687602 ) that the problem may be linked to the theme. Has anyone else had this problem?

If it's not the theme (I know it's probably not the theme) I suppose I have to start going through the plug-ins. Sigh.

John