Help with Error Unable To Fetch Response

Hello all, new user with a basic question. I have a KB word doc setup and when I run a test, it seems to hang and then stop and show error "Unable to Fetch Response". When I preview it and answer questions against the KB it works fine. I am using a Capture response with a last_utterance variable. I had the KB set using ChatGPT 3.5 and then changed to 4.0 to see if it made any difference but it didn't. Any advice on why its not returning a response? Thanks in advance.