Hi Rod, no I haven't deleted anything. It's just a standard quiz. Thanks for your information though. I created a blank project from scratch and inserted a question slide. The quiz review function works as expected, so it is something to do with the file I am working on. Possibly something that has occurred when upgrading from older version as it is an old file.
I will have a look at the quiz preferences again, but couldn't see what has changed. Will compare with my new project file.
Thank you so much for your help.