What are some common reasons why stretched canvas has wrinkles, and how can they be prevented or fixed?

Common reasons for wrinkles in stretched canvas include improper stretching, humidity changes, and poor quality materials. To prevent wrinkles, ensure the canvas is stretched tightly and evenly, use high-quality materials, and store the canvas in a stable environment. To fix wrinkles, lightly mist the back of the canvas with water and use a hairdryer to gently heat and stretch the canvas back into place.