Feedback for questions not working

Hi

I'm currently working on a module and have a lot of question slides. I've enabled 'feedback by question' and added bespoke 'correct' and 'incorrect' slide layers and deleted the default ones.  it's working brilliantly for the majority of slides, but i have one chunk of them which just won't work when I preview them.  They all have the same properties as far as I can tell.  A couple of the same design and content work so I don't think it's that.  Any ideas?thanks

Jessica