If you think that pictures are missing, please verify they are in the time range you have chosen. If this is not the case, you may have forgotten to check some types of posts ("Choice of the content for the timeline" section).
If you are creating a book for a specific friend/page, some photos may be limited by their privacy settings.
And at last, there is also the very rare case when we are simply not given your entire content that we should normally have access to. Facebook may refuse to offer us some parts of your data, even if there is no specific reason.