Have you installed the 7.0.4 patch? I seem to recall there was a bug with objects on right-hand master pages that was fixed.
I already had that patch but all the same I went to the update window and updated the application to 7.5.2 - it worked perfect first time, issue resolved!
Thanks for your prompt advice! Very much appreciated.
Is the Black background in anyway overlapping into the left hand page.
Select the black background on the master page and go to Window>Object & Layout>Align
Set the align to Align to Page.
And align the object to the page.
Do the same for the left hand page.
See if that fixes it.
I don't know but sometims for me any slight overlap of items on the master pages seems to make them disappear on export to PDF.
Also - have you tried different PDF settings?
I received an InDesign file yesterday where the exact same thing was happening for the user. They also sent me a PDF indicating where it was happening. When I exported it to PDF it was fine. I asked them to try changing their PDF settings and it output fine.
I'm not 100% sure what the issue was though.
It's rather odd.