1] The common reason could be outdated drivers. It is quite possible that some rogue device driver may be preventing your laptop from going into the Hibernation mode. A hardware device like USB flash drives and USB mice can actually keep your laptop awake! The recommended solution would be to first see whether you are using the latest available drivers for your devices. If not, you may want to update your device drivers before you proceed. You may also need to update your drivers for your Video Card.
If updating your drivers resolves the issue, well and good, else you may try the next steps.
2] Open Advanced Power Options via Control Panel and confirm that Allow Wake Timer is enabled as shown in the image.
Windows laptop will not hibernate
3] Use the built-in Power Troubleshooter and see if it is able to resolve the issue. It will also help you if Sleep mode is not working.
4] Enter your systems BIOS and check if Power-saving states like Sleep or Hibernation have been turned off. If so, turn Power-saving states on and restart your laptop. To enter BIOS you have to press specific key while the system is booting. This key depends on the manufacturer, so pleases find out which key you need to press while booting. You will also typically see this information while your system is booting.
5] If you need to troubleshoot or know more about the Power Plans in Windows 7 or Windows 8, then you may need to use the PowerCFG command-line tool. This tool helps you in diagnosing power-management issues.