There is a limit in the number of fonts which QT can access — if you exceed that limit uninstall fonts which aren’t needed.
Fonts are only used if the OS makes them available to all users — be sure you are using that option when installing/activating them.
There is a Carbide Create/fonts folder you can access using About | Open Data Directory — fonts placed in that folder will be at the top of the font menu the next time Carbide Create is launched.