[GeoffR]

Is it only your sideloaded font that doesn't work in this book, or does it prevent you changing the main text to other built-in fonts too?

Edit: If it is only the sideloaded font that doesn't work, then did you remember to turn the device off and restarting it after installing the font?

Otherwise, if you have converted the book with Calibre then there are some things that can go wrong that prevent fonts, margins, line-spacing etc. being adjustable in the new book. This is a problem with the way Calibre creates the .css stylesheet.

But some books are also created by the publisher in a way that prevents the font for the main text being changed. This is really a publishing mistake, and there are a few ways to fix it:

The easiest way is to do an epub-->epub conversion in Calibre, and tick the Look&Feel --> Filter style information --> Fonts box. This is a very heavy-handed solution it as it will remove all the font-family directives from the book, and just use the one font that you select for all text. But if the book only uses a single font family, then this isn't a problem.

A better way is to edit the book's .css stylesheet, removing the font-family directives from the paragraph classes used for the main text and replacing them with a single font-family directive in the body. This way the book will let you select your preferred font for the main text, but still allow other other fonts to be used in other parts of the book, and you can get the original font layout by selecting the "Document default" font if you want. You need to know a ittle bit about .css to do this though. I can explain in more detail if you like.