Right button not clicky anymore
Right button does not make the clicky sound and it is not clicky. Also it will click with much less pressure than the left button
As with most mice, the large lever/button you push with your finger activates a very small microswitch. If this switch is damaged or defective, the actuator on it may get stuck in either the up or down position, or simply lose its tactile click.
If the button is just sticking, you may be able to correct the issue by opening the mouse's outer shell, and using a precision screwdriver to pry up the edge of the actuator (usually a white circle inside a square black shell).
Otherwise, a replacement will be necessary - the microswitches are cheap and fairly generic, but this will require basic soldering.