Quiz Review Button

I have used the Quiz review button feature in almost all my e-learning module with no issue...  however today it is no longer working :(

When I click on the "Review" button (created using the inbuilt feature) it opens the first questions' review (telling me whether it was correct or not; with the green/red bar on the bottom of the page - exactly as expected) however it then gets stuck there - the next button does not work, so I cannot see/review any of the other questions

Anyone know what is going on here?.... I don't think I have inadvertently set a trigger, or disabled something... at least I don't think I have 

I have not done anything differently to what I normally do

Any help with this would be appreciated (attached the quiz if someone wants to have a look at it!)