My book of 130 pp is consists of 2-p spreads within inDesign. The spreads ending at pp 125 and 129 are not separated from succeeding spreads. I'm guessing this is not a show-stopper since print-ready pdf is generated in single pages. But it is a bit disconcerting. Masters used are no different from those used throughout the book.
Could you show me how to force separation of spreads?
Can you show us a screen shot, including the pages panel showing these spreads?
Open the panel's flyout menu and make sure shuffling is turned on.