Mh could be many things:
First logical thing i would do is unplug every USB device connected to the Laptop because it might interfere with sleep.Its important to restart the system after you have unplugged the USB stuff-restart and see if it will go to sleep.
Hybrid sleep borked:
"Power Options -> Change Plan Settings -> Sleep -> Allow hybrid sleep -> Setting -> Off" to disable hybrid sleep.
Dunno about this one but apparently setting it to off will actually turning it on as some describe.
Sounds funny though
media running preventing the laptop to go to sleep:
System Control\Hardware and Sound\Energy Options\Power Management Settings
click the notification center on the taskbar (looks like a speechbubble)
click on all settings
Network and energy saving
there will be a clickable option on the right side of your selected energyplan, that says change energy saving settings, click on that.
Now click on advanced energy settings
Scroll down to the bottom of the list until multimedia... now set the first one to allow sleep.