Storyline file that locks in a quiz

I have a project that a small percentage of users have issues with. It is fine for the vast majority. Once they get to the quiz at the end, the screen won't let them submit an answer- the submit button isn't activated. It works for the vast majority of users. This happens on the first quiz question, which is a 4-item multiple choice question. Below shows the simplicity of the question and properties. Are there any other occurrences of this documented?