Why does iBooks cut off my notes?

I added a note to clarify a passage in one of my books, and to convert population density from German miles2 to km2. I returned once, and found the note cut off partway though. I thought I might have saved it after the first step, and forgot to save it after the subsequent steps, of the conversion, so repeated everything and rewrote the note. I returned again tonight, and found the note cut off again at the same spot. What is going on here?