For some reason this has started working now.
I've created a form that makes use of conditional logic.
It works ok with all but one of the sections.
If you go to the page at http://indie-film.tv/film-festival-submission, near the bottom of step 3 you'll see a section that says:
'Do you have a specific niche'
This should then open up a new section, just as the one next to it does - and works.
This works in the form preview.