Tabbing Accessibility won't recognise answer boxes

I've created a timed quiz where the learner scores points depending on how quickly they answer - some wrong answers disappear as time goes on, reducing the amount of points a learner can gain from each question.  Everything works smoothly, except from accessibility...

I've checked through focus order setting the 4 answer boxes at the top in order.  I've gone through and turned off anything that isn't relevant from being picked up by accessibility tools. But the problem is when you tab, it tabs to box 'A' and won't tab to anything further.  

Does anyone have any advice? Is it storyline going crazy or am I missing something?  Thanks in advance.

(picture below to show the tabbing..)