Registration page allowing people to register without answering all required questions

Hello, 

We have dozens of people who are registering for an online meeting who have somehow bypassed one of the required custom questions. 

When we generate a report, it shows 'No response', which isn't one of the answer choices. 

My colleague and I both have tried registering and it will not let us confirm and register without answering this question. 

Does anyone know what's happening and how to quickly fix it?