Is it a Topaz?

If it is a KF8 book with embedded font(s) the publisher fonts option should be available, unless there is a bug.

It is possible to NOT embed a font but rather call out a specific font via CSS like they do for web pages. I should think the Kindles would normally ignore that bit of CSS though. However, I guess it is possible that if that referenced font corresponds to a font native to the Kindle PW it might lock it in and not allow any changes to another font. If that is the case, then Amazon needs to modify the FW to allow user override.

EDIT: With Sony readers it is easy to add custom fonts to a folder on the reader then use the method I described above with CSS to cause the reader to use that custom font. I did this all the time when I wanted to read in a particular font. It is usually not considered cool for publishers to force a font on us, but some still do it on occasion.