An article shows no summary in search or on its topic page

It has a password, or a membership plugin keeps it for its members (MemberPress, Paid Memberships Pro, WooCommerce Memberships, Restrict Content Pro, Members, s2Member, Groups or Ultimate Member). The help centre shows none of its words, not even a written excerpt: its card on the topic page is the link alone, its excerpt is empty where your theme shows excerpts, and the help search lists it by its title. This is the same for members, because a page cache keeps what one visitor was shown for the next; its own page still shows members the article. In the help search it is found by its title and keywords but not by words in its body, so give it the keywords a customer would type. A password-protected article is not in the help search at all.