Go to Control Panel>Power Options>Choose What The Power Buttons Do. and make sure that the action for 'When I press the sleep button:' is set to sleep for both 'On battery' and 'Plugged in'. Next Go to Control Panel>Power Options>Change when computer sleeps>Change advanced power settings. and make sure the power plan that your computer is running at the time eg. High Performance (Active).>Power buttons and Lid>Sleep button action. Has 'on battery' sleep and 'plugged in' sleep set for your computers ACTIVE power plan.