What format are your fonts in? I believe they must be in the TrueType format (.ttf). Other font formats may not work.
Additionally, if you haven't closed HitFilm and restarted it after installing your fonts, they may not show up.
Finally, what OS are you using? If you're using a new OS (such as Windows 11) there's a possibility that user fonts install differently from Windows 10. Note that Win 11 isn't supported yet.