Follow the instructions below:
- Make sure that Contact Form plugin is installed and activated.
- Install any standard WordPress theme (Twenty Fifteen, Twenty Sixteen) to check if the contact form will be displayed.
- Check your PHP template, and make sure that the content of your pages is displayed through 'the_content()' or 'the_excerpt()' - in this case, WordPress functionality will work with the shortcodes correctly. If such function is missing, the shortcode will be displayed as a plain text.