You probably get a few false negatives out this, depending on how good you are at drawing people out. Some people would be thinking "this question seems easy enough that I should be able to solve it without asking dumb questions, but I'm so nervous I'm not thinking straight" and then just freeze up. You should be able to draw them out, though, as long as you're aware that the reason they're not answering is mostly because they're nervous. It sounds like you've done a lot more interviews than I have, so I'm sure you could handle it. It is harder when their facility with English is poor.