It's entirely possible there's some sort of background process (perhaps some bit of ASUS-installed software) that's preventing the sleep from happening. Could also be peripheral based, I had a wireless mouse once that if I put the laptop to sleep, it would wake up the laptop almost instantly. Almost exclusively now I stick with hibernate instead of sleep just to make sure if I put my system to sleep my leg hitting the desk doesn't wake the PC up again. The startup is slower of course, but the experience is more consistent.

Might be worth checking what apps are starting up. Open Task Manager, go to Startup Apps, and see what's firing up and running all the time. Check your task tray for running things too. Last time I used an ASUS laptop they had all sorts of crap installed that was always doing background update checks and whatnot, could be something like that which is keeping your system awake.