Knowledge Check radio buttons

I have created a knowledge check slide with a multiple choice question. The radio button for the correct answer is selected on the slide. When I 'Preview HTML 5 in Browser', and it gets to the knowledge check slide, the radio button does not work. I can click on the answers and hit the submit button and it shows if i'm right or wrong but the radio button does not turn on to indicate to the user that they have selected an answer.

Is there a setting I am missing somewhere?