Why side mute button does not work?

When you flip the switch, does the phone indicate silence mode has been enabled or disabled? It will typically vibrate, assuming you have vibration on and it should show an icon in the middle of the screen to let you know the setting has been toggled on or off.

If the setting isn't getting activated in the first place, I would double check and make sure the switch is flipping back and forth properly It should click in either position. Some cases interfere with the functionality of the switch. I have also seen debris get stuck on the inside of the toggle, so it doesn't actually switch properly since it can't get all the way to one position or the other.