In years past, Apple’s motion coprocessor has been used to silently and constantly track steps and activity. But starting with this year’s A9 CPU in the iPhone 6s and iPhone 6s Plus, the M9 component also listens for voice.
The reason your iPhone 6s ignores you when it’s face down or in your pocket, is the proximity sensor located near the earpiece of the device. As of iOS 9, Apple also uses the proximity sensor to prevent the display from turning on when the device is face down or in a pocket. This is done to extend the battery life. You don’t need the display to light up when it’s in your pocket or laying face down.
That same sensor also prevents the device from responding to “Hey Siri,” as you likely wouldn’t use the feature if your iPhone is laying face down or is in your pocket. (No one wants to see you talking to your crotch, gentlemen..) So, make sure your device is laying face up and is out of your pocket to insure a response from Siri when you speak to her.