I hope you are well today and thank you for your question.
Could you please tell me how you are trying to add it or share the screenshot of it and also share the page URL from your website where it is not displaying so that I can troubleshoot it?
I’m adding it using classic editor “more tag”, didn’t work with Gutenberg either, i even tried several plugins but nothing. even that more tag appears in the editor but nothing in the post page (.
attached are some screenshots and this is the link of one of the articles that I’m trying to add read more into it.
Solution for this topic
The read more tag only works on the blog page as shown in the attached screenshots.
You can also confirm it by using any default WordPress theme such as Twenty Twenty One theme.