We have a project that the slides won't advance. There aren't any click boxes or other objects on the slides besides the submit button on the one question slide at the end.
Did you check the master slides?
What about the Master slide? I didn't think they allowed click boxes or buttons.
Shape buttons are possible on master slides (since version 6, more than 7 years ago). Since 11.5 even SVG or bitmap image used as button can be on master slides.
Another possibility is having a Pause command On exit for slides, or On Enter, as is the case for lot of slides in the Quick Start Projects provided with 11.5.