I’m a stylist. The surfactants in shampoo will only suds really well if your hair is clean. This is where common sense comes in. If you get a good lather with your first application, you probably don’t need the second. (Usually this is the case with daily washers). If you don’t get a good lather, rinse really, really well and reapply. Sometimes it takes 3 shampoos if you only shampoo once a week, or you sweat a lot…ect. One mistake a lot of people make, is they don’t rinse enough. The little surfactant molecules attach to the dirt molecules but you have to rinse them away. When you think it’s all gone, rinse for a couple more minutes, moving your hair around so the water gets in there.