Dirt/dust remains when wiping forwards, backwards and forwards again. Is the appliance only intended for forward mopping or is there a fault with the appliance?
It could be one of several things: 1) The area is particularly dirty. 2) The water tank may not be full and the brush is running dry. 3) You may need to clean the appliance (manually, not just on the base). This is because the appliance contains a metal scraper that traps a lot of hair/dust but which settles there. You have to remove the brush roller and clean the area with the brush supplied. From time to time, I also like to clean the brush manually with a detergent and rinse it well. Then I place it on the base to dry.