Might be an application not shutting down, try enabling auto shutdown, see
here.
You can also amend your "WaitToKillAppTimeout" and "HungAppTimeout", see
here. The only constraint that you must follow is to make sure that the HungAppTimeout is greater than the WaitToKillAppTimeout.
WARNING!!: The above involves playing with your registry - so make a system restore point before messing about with it.