Originally Posted by Hendrixxxxxxxx
I think you'll find that the problem you describe is only evident with azw3 books.
I don't understand all the technicals, but azw3 uses a different "system" (ie, not Java) for displaying the books that you are reading, and hence you are limited to the default font sizes, no matter what sizes you have specified in JBPatch. If the default sizes are (making up numbers here) 7, 9, 13, 16, 25 and 31, and you specify 9, 11, 15, 21 and 31 in JBPatch, the only ones you'll be able to use will be 9 and 31. Trying to select any of the other sizes won't do anything.
If you were to re-encode the books you're having issues with as mobi files, you will find that all of the font sizes you have specified in JBPatch will work.
The other alternative is to use something like calibre and embed the font sizes in the book this way. I have not done this, so I don't know if it works or not. I simply convert all my azw3 files to mobi (using calibre) and I have full control over the font sizing on the Kindle.
I encountered the same problem as mentioned by Johnleo. it happened to azw and mobi books. I remembered reading in this forum that this is a known problem, and it is caused by the fonts used.