Some applications can prevent computer from sleeping even when you are not actively using the computer. To find out what applications are keeping computer from entering sleep or other low power mode perform following:

• Open elevated command prompt
  Start > All Programs > Accessories > right click Command Prompt > Run as administrator
• Type powercfg -requests and press Enter

This will lists all applications and drivers which have made a power request to prevent computer from entering low power mode. In the screenshot example above you can see that computer is prevented from sleeping because of TeamViewer remote access session and playing audio stream.

Windows 7