The font shows in font family on kindle but doesn't work

[n3nuzzzz]

Hi I just uploaded Fast sans and Fast serif on my kindle but they are the only 2 fonts that don't work. I can see them in font family but when I choose one of them, they don't work

[Born2Root]

Hi,

like mentioned on the main Insturctions for Kindle...
It seems Kindle handles fonts different depending on the source of the books (bought from the store vs. loaded locally from Calibre).
If you are having trouble to get the Font working on Kindle, please refer to the Github Issue

Please be also aware that only the new Kindles are that easy to handle, where it is enough to load the font onto the device.
If you have an older Kindle (below 5th generation) please refer to the Github Issue

There was a user that wrote a little python script to apply the fast-reading feature directly in Calibre. That can also help on older devices.
https://github.com/giutala/calibre_bionic_converter