Simple pull quote was working fine for me on my website before the latest upgrade of WordPress to 5.3 The former posts where I used pull quotes are still appearing fine on the front end, but in the editor, there is no sign of Pull quotes… as an option. If I just use the shortcode [pullquote] Lorem Ipsum [/pullquote] – this also works fine. Try that. it might help even if now, with the latest version of WordPress being 5.3, the pull quote plugin is not loading properly with the editor.
