I have had this problem with laptops in the past, You will find it is not the actual tactile button on the board but the plastic cap that is mounted in the case above the actual switch, what happens is wear in the plastic and the button slides sideways slightly, if you look carefully you will most likely see that it is not central in its socket, you can try a pin to move it over and centralize it again, then in future you have to be very careful to push the button straight down as any slight sideways force will move it over again. On some laptops the button bar will just pop of with a small lever (a screwdriver) if your's has a separate bar like this you can remove it and re-centralize the power button there is not normally a spring for the button return they rely on the spring force generated by the tactile button on the board which is often hardly enough force to lift the plastic button.