[Rich Gibson]

I believe I may know the reason. When I tried to get to reader app to play the document with voice it started spelling words instead of putting them together. A careful review using Apple's Preview shows the scanned print is significantly fainter than every other scan. The book does appear light to the eye as well.

I tried the pdf file on the tablet and it reads better...clearly the first scan wasn't good enough. I rescanned with a darker setting and tried a few pages and copied it to the tablet and the voice speaks the pdf file perfectly but somehow Calibre produces a mobi document without any text....but it IS in the pdf file before I convert it. Go figure!