When your phone is facing down and the screen is off, the phone will not allow you to use Edge Sense
This is to prevent you from accidentally triggering the squeeze gestures when you lift your phone from a flat surface while it's facing down.
If you need to hold the phone facing down to capture something high above you—such as the sky or high objects—but squeezing won't take the shot, try tilting the phone's angle to about 45 degrees and then squeeze the lower sides of your phone.
Note: Future system software and Edge Sense updates will allow you to use squeeze gestures while the phone is facing down and the screen is on. This feature will be available in the preloaded Edge Sense on newer HTC phone models.