Are you able to send me a live link/URL to your search page so I can take a look? Can you give me a term to search for that will yield results.
If you are using the ‘As an Archive’ method, and don’t have a results page slug set, code LIKE this might do it (the ID number shown would change to your form ID:
function sandf_change_search_url_rewrite() {
if ( is_search() && ! empty( $_GET['s'] ) ) {
wp_redirect( home_url( "?sfid=1234&_sf_s=" ) . urlencode( get_query_var( 's' ) ) );
exit();
}
}
add_action( 'template_redirect', 'sandf_change_search_url_rewrite' );
This needs to be inside the child theme functions.php file. You would need to be using a child theme, otherwise any theme update will overwrite the changes. Orbisius have a child theme maker plugin that is really simple to use (just accept the default settings, after done, remove the Orbisius plugin).
https://wordpress.org/plugins/orbisius-child-theme-creator/