To parse .html or .htm files as .shtml add the following to your .htaccess file in the public_html folder:
AddHandler server-parsed .shtml .html .htm
If includes are still not working it may be the case you are on a server with nginx reverse proxy setup. If so, contact us and we can place an exception on your domain name for it. It is highly recommended to use .php or .shtml instead and utilize the nginx reverse proxy and varnish cache as they can speed up your website significantly.