Rugs, Carpets and Carpeting - carpet stains at interior door threshold

My home is only two years old. However, the carpet has already developed a darkish stain at the threshold of each interior door, exactly where each door is positioned when closed. My first thought is that the stains are caused by dust particles that are carried by the return air that blows under the doors when the air conditioner is running. But, I have tried carpet cleaners like Resolve without success. If they are just dust stains, I would think that Resolve would work. But, nothing seems to work. The doors are made of Masonite. Can anything leech from Masonite? Any suggestions?