[GeoffR]

One thing to check is that you don't still have an old version of the font on the device. The ePub reader can select the wrong font if there are two fonts with similar names.

e.g. I have "Courier Prime" which works fine, but if I add another font called "Courier Prime Sans" then I can no longer select the original "Courier Prime" in the ePub reader, I just get "Courier Prime Sans" instead when I try. I would have to rename them something like "Courier Prime Serif" and "Courier Prime Sans" if I wanted to be able to select both versions of the font. (And by rename, I mean change the internal names with fontforge, not just rename the files.)