Font is installed in font book, but bold does not work I've installed a font in the font book, but the bold format does not work. I see the bold in the font book. Also, the italic format does work.
How to fix it?
(I'm running on El Capitan).

Is there an option to reinstall the font?
Your font could be broken. Not likely if it worked before but at least it is easy to check as there’s a font validator built into Font Book. Please mark the font and select File > Validate Font. 

If you receive warnings or errors there’s no built in repair tool, though. But then at least you know...