The number one cause of this is using paint of poor quality. Other factors responsible for such an appearance are using sub-standard painting tools and improper painting technique. Painting with a poor quality roller cover or a brush with harder bristles can cause this problem. Touching up when the previous coat has dried or is about to dry can cause poor leveling. Brushing or rolling more frequently than necessary also can cause this problem. Painting a warm or porous surface makes the paint dry faster and can cause poor flow and brush marks.
Always use good quality latex-based paints which contain the right proportions of binder and thinner. These paints flow and spread uniformly. Due to this the brush marks or roller marks are automatically leveled out before the paint dries into a film.
Different types of paints require the use of different lengths of roller covers. Therefore, always use the nap length for the roller cover as suggested by the manufacturer. Use a brush with high quality bristles.