wannabee: Thank you so much for your response. After I read your advice, I checked to see if my images were over the maximum pixel size of 2m. They were, so I went back resized the images, and changed all the code.
As these are picture books, I don't want to reduce them too much, but I made them all under the file size limit that Apple stipulate.
Unfortunately this didn't make any improvement - however it was well worth a try.