Sometimes this can be remedied by installing an updated driver for your display adapter.
My Dell Latitude D610 laptop has no problems going into standby mode at 1 minute. Anything over 1 min, it will refuse to go into standby. I have the latest drivers, and bios. Bios settings support standby modes and is enabled. My LAN port is disabled, I only use wireless. I have no problems with hibernation.
I have not had this issue before until several weeks ago. I am not sure what happened (maybe some type of update happened without my knowledge).
My power options are normal (set to the default laptop/portable option), and I can manually put the laptop into standby and hibernate without issues.
Does anyone have any ideas??