CSS ::after and ::before not working

I cannot use ::before or ::after to add font awesome 5 icons and it is incredibly frustrating. I copied the EXACT css that this plugin uses and applied it to an element using ::after. It will not work at all. This seems like a major oversight – why would you create a plugin to add font awesome icons if the most basic way to add them via css doesn’t work?

I need to be able to add icons via ::before and ::after selectors. Please advise.