Computer mice are prone to not working correctly if they get dirty
. Dust inside the mechanism can play havoc with a mouse's function. The easiest way of dealing with this is "mouth to mouse resuscitation", as I have heard this process described. Simply blow on the ball part of the mouse. This usually manages to get rid of any gunk. If you have an infra-red mouse mechanism, check from time to time that the lens is clean and dust-free. Blowing off the dust can work quite well, but if you can see stickier residue, use a cotton bud dampened with meths or white spirit (or vodka or even vinegar), or else a microfibre cloth to clean the lens.