I have found that ensuring "automatic updating of links" is enabled solves that problem. I had this happen before and nothing was being written to the responses.xls file...this one change solved it for me. For what it's worth, I was on a more secured network where it may have played a bigger role.

You may also need to adjust your security level (ie: go to the options section of excel, and add to your trusted locations...the directory in which the file is being referenced). I think medialab creates a temporary "responses" file before outputting the final version after running the experiment.

Hope that helps!