There are four cause of this that I’m aware of:
1. You havent turn on the font awesome fonts.
It is located on: Appearance – Costumize – Advanced Options – Front End Icons
(Font Awesome)
2. You are using adblockers, try to turn it off, all of them. If that’s the case, set the adblockers configuration to let the Font Awesome loaded.
3. You are customizing your .css file, either on child theme or main css assets. Do theme reinstall. Try emptying your function.php, custom css, or other customization.
4. It is possible that other plugin causing it. Deactivate all of your plugins, check whether the problem solved, if it is solved, then reactivate the plugin one by one. Find the problem.
Hope its helps.