DIAGNOSIS: You're not breaking it, but the joystick/navigation button on the back can be finicky. It's a multi-direction press: up, down, left, right, and a press-in to select. Stiffness or missed inputs usually mean you're not pressing directly in one of the four cardinal directions.
FIX: Use the pad of your thumb, not the tip. Place your thumb squarely over the center of the joystick. To move up/down/left/right, apply pressure by sliding your thumb in that direction from the center, don't try to push the edge of the stick. You should feel a distinct click for each direction. To press-in (select), push straight down from the center. Practice navigating a menu you know, like Picture Mode, to get the feel.
IF THAT DIDN'T WORK: There's a small chance debris got into the mechanism. With the monitor off, try gently wiggling the joystick in all directions and blowing a short, sharp breath of air around its base to dislodge any dust. Don't use compressed air cans, as they can force moisture inside.
HEADS UP: If the joystick is physically stuck or only works in some directions no matter how you press, that's a hardware fault. Contact LG support while the monitor is under warranty. Don't try to pry it open yourself.