Unfortunately the Thumbsticks break after some years of heavy usement. Buying a new one is expensive and not necessary, as you can exchange this part if your joypad is technically still ok.
The sticks are the same for Xbox One, Xbox One X and Xbox One S. We offer here two thumbsticks in one pack. To exchange it you need a small screwdriver and patience. If you do not have a screwdriver, you can get one on our shop as well.