White screen when pinching

Dear All,

I have just finished my iBook and i'm watching it in my iBooks on my iPad. All works good but sometimes when i want to open a page by enlarging it from the thumbnail viewer in the bottom, i get a white screen. than when i open other pages they are also white. When i close the screen and open it a few times, it gets normal again. What can be the problem?

Best regards,

Vec