Do as S.Sebgupta and Nepatsfan have said. In addition....some software
programs such as Norton Antivirus are notorious in limiting the time that
you can choose (when I had the Norton Antivirus installed, my limit was 25
minutes . Anything over that and my PC would not go to Standby). Try it with
one minute (it's the minimum) and if that works then you know that it has to
do this limit. If it works with one minute, increase this time until you
reach some possible limit. If, on the other hand, the one minute test does
not work, you may want to look to the other most common culprits where the
Video drivers seems to be in the 1st place.