Women Painters.com: Paint Issues and Fixes:

Poor Stain Resistance

Failure of the paint to resist absorption of dirt and stains.

image courtesy of
The Rohm and Haas Paint Quality Institute

Possible Causes:

Use of lower quality paint that is porous in nature.
Application of paint to unprimed substrate.

Solution

Higher quality latex paints contain more binder, which helps prevent stains from penetrating the painted surface, allowing for easy removal. Priming new surfaces provides maximum film thickness of a premium top coat, providing very good stain removability.