These features based upon sensitivity can sometimes require tweaking and updating by the manufacturer to iron out problems.
When I first got my S3, the Smart Stay feature worked only intermittently, and I found myself staring at the screen with bulging eyes trying to keep the screen from timing out. Then the first software upgrade improved it and the feature worked perfectly from that moment onwards.
It can be a matter of needing to adjust the tolerances for variations in user gestures to make sure it works in nearly all cases easily.
The palm-swipe to take a screenshot still demands a bit of skill...