It may just be that there are no system updates available for the phone. What carrier are you on, and was the phone originally purchased from this carrier or from a different one?
The phone is bought from O2 DE but i always used Vodafone. It used to work until 1 month ago. I bought another samsung ,not a s6, at the same time and place and it works fine.
A carrier-specific phone usually has to be on that carrier's network in order to get the proper system update. Or you could try using Samsung Smart Switch.
At the moment seems there is no new updates from samsung. I don't think that is a carrier problem because i have used Vodafone since when i bought it and it updated to Marshmallow. It only bothers me that there is no reactin from the commands and as I searched on the net no one had this issue.