Or, tap the app icon and do not reopen it through the Recent apps list to make sure squeeze gestures work in the app.
If you want to ensure your phone won't trigger Edge Sense, you can turn the feature off whenever you need to use a car kit or selfie stick.
When the screen is off and the proximity sensor is blocked, the phone will not allow you to use Edge Sense. This is to prevent you from accidentally triggering the squeeze gestures when you grab the phone out of your bag or you're holding it close to your ear during a call.
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.