Hi,
Make sure that the automatic replacement is activated. If that does not work, then the theme does not use the recommended get_search_form method to display it’s search bar, thus making it impossible to replace automatically.
In that case, usually the only option is to change the shortcode within the theme files directly. For that you can ask the theme author, they usually know how to do that.
You can also add temporary FTP and back-end details, and I will check the theme files, maybe I can find the corresponding file to replace the form.