I had the same problem a couple days ago, now I figured out what was wrong. I was able to manually put the system into standby mode, it means all the hardware and their drivers support suspend. The problem must be something else. I noticed that if I rebooted machine but didn't log in, the system was able to go to standby mode without problem. If I logged in, it refused to go to sleep. So I figured that the problem is due to some startup program. I have one airlink printer server program installed which was started after I logged in. So I just removed it, and reinstalled it choosing not to put it in startup programs. Now my machine can go to standby mode automatically. I would suggest looking at the hardware/software that are not bundled with Dell.