I have had some instructors run into the same problem and found one common point in many of these cases. Make sure the previous debriefing screen has been closed before running the next scenario. When we have had instructors forget to close the debrefing session and had it running in the background, we have found the Instructor Interface will freeze sometimes. If that is not the case, it is always a good idea to reinstall the SimMan software. When the program starts acting funny, thay often fixes it.
Dave Rodgers