Well, that was interesting! A customer notified me recently (thanks Peter!) that he was unable to adjust the font size of Wallis Budge: Magic and Mummies in London and Cairo (Rev. Ed., 2021) and that the text was too small for him to read comfortably.
When I discussed this with Kindle Direct Publishing, I found that the text had been published in what is called "fixed text format," which is supposedly better for illustrated books, but which is (as the name suggests) a fixed format. Thus, I needed to unpublish the old version and republish a new version of the text that is "reflowable"--the font is able to be adjusted according the needs of the reader.
Of course, once I decided that I needed to revise the text, I had to contract with a company to reformat it. Flatworld Solutions in Bangalore was very responsive and did excellent work reformatting the text as a reflowable file. I then hired someone on Fiverr to add a clickable table of contents and the new version was ready to go.
KDP says that they will notify those who already have the kindle version that the reflowable text is now available to them to replace the fixed format version.