Check boxes not appearing in display

Re: multiple choice questions.

Several people, over time, have reported that check boxes do not display. I have reviewed all of those and they do not relate to the problem we are having using Quiz 7.x-5.3 on D7.6.

I have installed the latest version of Webform and in that check boxes do appear.

Some have, in the past, suggested that the problem is related to themes developed in Bootstrap. In our case, it is not. I've tried our custom themes, third party themes and all Drupal 7's basic themes using a mix of Bootstrap and non-Bootstrap themes and the check boxes do not appear in any of them.

We are running this installation on PHP Version 7.0.32 but I've tested on other 7.x versions and on 5.6 with the same result.

I've looked at the source of the display file and there is no mention of the boxes:

------

<tbody>
 <tr class="odd"><td width="64"><span class="quiz-score-icon wrong" title="Score if chosen: 0 Score if not chosen: 0" />
</td><td> <p>One</p>
 </td> </tr>
 <tr class="even"><td width="64"><span class="quiz-score-icon correct" title="Score if chosen: 1 Score if not chosen: 0" />
</td><td> <p>Three or more</p>
 </td> </tr>
 <tr class="odd"><td width="64"><span class="quiz-score-icon wrong" title="Score if chosen: 0 Score if not chosen: 0" />
</td><td> <p>Two</p>
 </td> </tr>
</tbody>

-------------------------

We are running this installation on PHP Version 7.0.32 but I've tested on other 7.x versions and on 5.6 with the same result.

I have tried using Display Suite and without using Display Suite. The result is the same in each case.

I have tried putting the question into a book and tried as a stand alone page. The result is the same in each case.

I have not modified any Views.

Does anyone have any suggestions as to how to fix this?

Thanks