Once again, the sun plays havoc with everything it touches. The sun's ultraviolet rays cause a molecular chalking effect that is completely normal. Noticeable chalking can be controlled.
If you wash your siding twice a year, it will maintain its luster longer. Washing vinyl siding consists of spraying the siding with a normal garden hose that has a siding washer attachment connected to it. A siding washer attachment is simply a bottle of chemicals, like car wash detergent, formulated for siding.