It (they) may be missing, or you've installed conflicting third party fonts. I would do these two steps in the order presented:
1) Clear all font cache files from the system. Close all apps. Open Terminal and copy/paste this line into the Terminal window:
sudo atsutil databases -remove
Enter your admin password when prompted. This will remove all system font cache files, and the font cache data of the logged in user. Immediately restart after running the command.
2) If that doesn't do it, open Font Book and run the command, Restore Standard Fonts. This will move all third party fonts out of the three main Fonts folders on the drive (System, Library, logged in user) to a new folder named Fonts (Removed) next to each of the three Fonts folder locations.
Only the fonts installed by the OS will now be active. If that clears the issue, some third party font you installed was conflicting with the intended font.
These are the basic fixes. If you tried to replace the San Francisco system fonts with a hacked version of Lucida Grande, get rid of these hacked fonts. All they're doing is creating intentional font conflicts.