It is a little complicated to explain, but pages exist in Pages' Word Processing documents only when there is text on them, whether that is visible or invisible characters (such as returns, tabs, page and section breaks).
It sounds like you have a mix of returns, spaces and tabs on your pages that you are using to position objects, which in turn have text wrap on them, that push text away from them.
The text fits around the nooks and crannies of the objects, but when you move one or more the combination shuffles and text or invisibles that popped over the page will suddenly fit back onto previous pages and leave the pages that have objects, but no text, behind. They are still there, but not showing. They will reappear if you insert page breaks or anything that will recreate the foillowing pages.
If this is a problemn for you, or you are still learning Pages, work with Layout templates, selected from the Temp[late Chooser, which are all isolated pages and which do not behave like the Word Processing pages.
Learn a bit more about Pages here:
http://www.freeforum101.com/iworktipsntrick
Peter