I changed the page size and margins of a document, redid the master pages, etc. Document setup: facing pages. Margin and alyout tab: facing pages. Master pages: spreads. The resulting document: all left-hand pages from the A master. Half the margins are wrong and the running heads don't show. What happened>
I marked this correct because it led me to the solution, but it seemed to be a little more complicated. I had to also select pairs of pages and 'allow spreads to shuffle.' Probably there was an easier way, but thanks!
I"m glad it led you to a solution. Thanks for adding that information. It will be helpful to others who might have this issue.