In general - never *ever* filter someone out on one part of the interview. Except... if they go silent - instant fail. if they say to me - sorry, I can't think right now / could i have 5 minutes alone / could i get back to you later et cetera, not such a bad response.
Going silent - always bad. If someone doesn't understand, doesn't know, needs help, and they don't tell me - then they don't fit on my team. Maybe they are great at their job, maybe they are awesome at some things, too bad, just too big a risk I'm afraid.
BUT - that is because of my personality as much as theirs - I need people who will point out when they have a problem. Interviews aren't just about the candidate, they are about my team, the project they will be going to, the people they will be working with and me (assuming I will be managing them).