Bob Levine said:Another quick fix if you're experiencing this issue is create a new book and bring your InDesign document into it. Export the PDF from the book panel which doesn't work in the background.

For me, a great bit of advice. Worked like a charm thanks. I'm using a Mac and often the export doesn't run at all in the Background Task mode. I read elsewhere that one should try having the Utility open and run the Export through the Adobe pdf preset drop-down. This only seems to work with one export. Come the next one I have to Force Quit and start again. Bob's advice has helped me to avoid all that.