If you did not clean your paint brush properly the last time you used latex paint it may still have paint in it and it will be stiff. To fix a stiff paint brush pour vinegar into a glass container and put it into the microwave until it is hot but not boiling. Pour the vinegar into your cleaning bucket and throughly soak just the bristles for about an hour. Wash the paint brush throughly with soap and water and your brush should be in much better shape.